The 30-30 in the contender is a great cartridge. Another vote to leave it alone.
If you are bound and determined to change it, you have but a few choices. 30-40 Krag was mentioned before. You can also go 30-30AI and bump out the shoulder. You can also go with a 30 (censored word) (kinda krunch the last two words together.) Uses 308 dies but 307 rimmed brass. You can shoot 308s in one, but they actually need a different extractor.
All these mods will cost you about $100+ and you gain maybe 200fps advantage. 125 Nosler ballistic tip will give you more fps out of the 30-30 and is great on deer and antelope size game. No extra charge.
Steve