Re: Argentine Mauser
the 1891 is an excellent action for a custom build. Any quality gunsmith could make the action functional again (assuming the piece is "missing" not been damaged beyond repair).
google up LOSOK arms, he's a mauser expert and can fix you up with about any build style you like for mausers. You can go tactical (mcm will inlet any a-series for mauser 98), hunting, classic, safari style, scout rifle, military original look, all kinds of things.
My buddy has an octagon barrel 270 done on an argentine action. It's got beautiful birdseye in the wood stock... looks like something a rich dentist might own for a safari, lol. This thing will shoot bugholes, it's more accurate than my tactical rifle. No idea who built it, rescued it from a pawn shop for a couple hundred bucks.
so, FWIW, i'd have it repaired & rebarreled to something modern and shoot it! The argentine is a desireable action to start from, and the mauser design has all the good stuff going for it: controlled round feeding, claw extractor, etc.
the 7.65 uses the same bolt face as all the cartridges in the .308 or .30-06 families, sou you can chamber in those without having your bolt modified. If you have the face opened slightly you can do 6.5x55 sweede.