That info can be found mostly in the writings of the various 'gun cranks' that were building Varmint guns when that was the thing to do.
The 44 1/2 model is pretty close to the strength of an 1885 Winchester but it has a smaller barrel shank, so it is limited in top end size cartridges. There is a company making new repro's and that have made the barrel shank (and, IIRC, the whole action) slightly larger to take a wider range of cartridges.
If the breech block drops straight down, it's a 44 1/2. If it pivots as it drops, it'd be a 44, and not so strong.
If you want a book reference, look for Frank DeHaas' book Single Shot Rifles and Actions. It covers some of the history, a lot about caliber conversion options, and a fair bit of what would be considered butchery, today, of old guns that nobody really wanted at that time. Worth digging out a copy for a read.
If it is a 25-20 Winchester caliber, brass is quite easy to get. If 25-20 Single shot, not as easy, but available, and not so expensive that you couldn't afford enough to set yourself up.
If it is one of Stevens' proprietary cartridges, then you are looking at even more money for cases, but there are ways around the problem, between buying custom turned cases made one at a time, or learning to make good use of the few that you dig up at shows and the like.
I read one gent's post that he had shot an entire season on just two 25-25 Stevens cases (They look like a long section of a pencil, nothing else like them to form brass from) by using one case with a dowel in it, to breech seat the bullets, and loading the other case as required. He had planned to keep shooting the one case until it was too far gone to be reused, then swap them around, using the used case to seat the bullets.
Cheers
Trev