I have one of these potential money pits as well ...... and was wondering how much work would be needed to make anything other than a "whatever x57" based cartridge feed properly.
The action is not strong enough (46000 CUP limit) to run full power 7 or 8x57 loads in ..... there is always the traditional answer of .257 Roberts...... but I think the original 7x57 Mauser is superior to that.
7.62x39? Would that feed though the original internal magazine?
TheGolden Rule of Tool Use: "If you don't know what you are doing, DON'T."