I have had to tackle the same issue for the last few years as well with a bunch of old leverguns & a couple SS's. I hjave used a # of different approaches from elegantly & tastefully done to rudimentary design& functionality... both will work but how long your shots are & how fashionable you want to be in the field is your choice. One thing is certain after a # of trials& errors, with a straight stocked rifle that is compatible with the original barrel sights, a cheek riser is essential to good scope usage success. I have use 18 and 32 inch brass tube scopes, modern bolt rifle scopes from fixed power to 6-24 and am now mostly using red dot's on all my 1800's lever guns but they could be used on a Sharps very easily as well...they "aint perty" but they work and keep me being able to shoot my old guns...hell, I've even mounted a bow sight on a Marlin Cowboy and can shoot "hunting accuracy" out to 300 yrds with it without changing "cheek weld" from 50 to 300 but that should be a different thread all together.
As there are a lot of us "old bastards" in the same predicament I will take the time to offer up my experiences with my experimenting.
Brass Tube scopes; I have two, an 18" with dovetail block mounts and "Unertyl" style adjustment ring at the front mount, and a 32" with the "Malcolm style"(currently mounted on an 1871 RB) vernier mount ( the original mount base was a "screw needed" type but I manufactured a dovetail mount myself). Both are 4X. Both mounting systems work satisfactory, the Malcolm is heavy but somewhat easier to "O" than the unertyl style. The Malcolm has a very much larger adjusting range if very long range shooting is your game. The thing with both these scopes is that they have no "parallax" resistance at all, they are simple and just a tube with glass & a solid cross hair inside, it is for this reason a solid cheek rest & weld is essential to repetitive accuracy...no mater how you sight the gun in , if your neck posture/line of eyball sight through the scope changes point of impact will always change also. I have never used these scopes for extreme long range but have killed deer with each of them from 80 yrds out to 245 with a 45-70. They have limitations but with some attention to detail they work admirably.
Modern scopes; I have mounted modern scopes on !890's Marlin & Winchester lever guns & modern Sharps/RB actions with success as well but when I tried to market the mounts I met with all sorts of derision from wholesalers. I will admit that they "weren't perty", a bit cumbersome but they worked and I have shot 200 yrd 3" cast bullet groups with a # of 130 yr old rifles using them...groups that I cant even see the bulls-eye at that range without magnification.
The problem with easily mounting modern multi-powered scopes to a tang rifle is that they are heavy and, although a single point attachment to the original tang screw holes works sufficiently with operator handling considerations, a double point mounting is superior. Because of the simplicity & convenience (everybody should have an old scope kicking around) and the fact that shot distance is only regulated by your own skill I think this is still the best option in use but not marketability LOL. Another drawback is that all old Winchesters are top eject and a scope over the ejection port sort of messes with the "rack& throw" option...it basically turns them into a single shot and brass have to be finger-picked out of it...but I was still shooting them accurately and that's what counts in my book.
"Dot scopes"; This is my current "spearmint " and is going quite nicely if 125 yrs or so is going to be your limit on a deer. I should temper this by saying bot the dot scopes I have are of the "inexpensive variety that have no magnification at all, I have never looked through one with magnification but understand they are out there. The convenient thing about these scopes is that they are very light & recoil doesn't bounce them around on the mount the way a much heavier modern scope is. For this reason mounts can be very easily made out of aluminum and single-point mounted to the tang the same as any vernier sight is. For this reason the mounts are very easily made with apiece of alum angle, a hacksaw and a hand drill, the only thing that most folks will have to by is a piece of Picatiny rail blank( You only need 2-2.5", easily found in Canada). I am lucky in that I have a shop fairly well equipped with hobby machining tools and can build a mount in an hour but someone with the tolls mentioned above could easily cut one out in an afternoon. So far I have used these "dot scopes" on paper & "Cowboy" sillywet targets out to 100 yards and they work well ( paper at 100 requires at least a 6" bull to be seen)... by "works well" I mean very much better than even a vernier sight at that distance for my vision quality.
I haven't use the dots for any hunting yet but plan on using my 38-55 with one this year for deer. I just spent a very enjoyable afternoon yesterday with a heard of Mule Deer feeding around the house for a couple of hours. I stat inside, using a chair back for a rest and just tried both dot scopes out at different ranges on those deer. they varied from 60 yard out to 125, out to 125 with a rest, I would have very confidently "rib hit" any of those animals. Out a bit further than 125, I could have "front 1/4" shot one but the certainty of not hitting a front shoulder and ruining a pile of meat would have kept me from trying the shot. one thing I will mention if you are thinking of a dot scope, one of mine is just "red" and the other is red or green, shooting at white paper or metal targets it doesn't make much difference but while scoping those deer in different lighting conditions, I never buy another with "just red" as the green is much more usable on any type target.
This is long enough and I will close with again mentioning the importance of an elevated cheek piece for any quality scope use on a straight stocked rifle.
...one more brief point, I will not modify or change any aspect of any of my "original rifles" to mount a scope.
