I'll take this one folks, as I have asked it a few times myself
To summarize:
If you want to scope a mosin, the easiest and least intrusive way is to buy one of the PU repro sniper rifles already for sale. That way you don't have to drill holes in another rifle and destroy it's "original" collector value. Yes - it's a Mosin - but the same was said for the Lee Enfield's for sale in the barrels at Canadian tire
They are quite affordable right now, and westrifle has some for sale as I type this.
If you don't want to go that route, you can always buy the repro mounts and scopes, both PU and apparently PE/PEM scopes eventually, and have someone who knows what they are doing do the install. At least this way you are installing a reproduction of equipment that may have been used on your rifle, depending on the year of manufacture.
A no-gunsmith option is available for scout scope mounting. This replaces the rear sight leaf and requires a long eye relief scope. Not a popular option from what I have seen, but people have done it, and some people like it.
And last but not least, westrifle does offer some drill and tap mounts that will allow the mounting of a modern scope to your mosin.
Hope that helps.
For what it's worth, I too wanted to mount modern optics to my mosin. I eventually got the PU sniper repro from westrifle and it has quickly become my all time fave rifle.
Good luck and enjoy the mosin!