It is a Model 1893 Spanish Mauser, and it is small ring as they all were up to the Model 1898. Brownells has pre-threaded barrels for that action in several calibers in the same pressure range as the original 7x57, including the 7x57. I strongly suggest not trying for a higher pressure cartridge, as not only was the original design made for cartridges in the 40k-45k psi range, but Spanish-made receivers are notoriously soft.
The type of scope mount is based on what you like and can pay for, as is the scope. You will have to turn down the bolt handle or cut it off and weld a low scope type on in its place, and also fit a low-scope safety. Drilling and tapping can be done on a milling machine or with a drill press using a jig. Free-hand drilling will result in a ruined rifle. This and other work you might want to do or have done, like a new stock, rebluing, etc., will be quite costly either to have done or to buy the equipment necessary to do them yourself.
The bad part is that the rifle is one of the less desireable Mausers, so that when you are finished and have spend a fair amount of money (the barrel alone runs some $225), the rifle will bring around $150 on the used gun rack at the local sporting goods store.
If I may be honest, your questions tell me that you have not tackled anything like this before and may be getting in over your head. Might I suggest you forget the project and look around for a good used sporting rifle in the caliber you want?
Jim
