A .35 Whelen re-bore would be really cool. An iconic cartridge in an iconic action. Should be pretty easy, as you wouldn't have to open up the bolt face, and the dimensions should work everywhere else too. Brass and bullets are fairly common.
If you went with a re-barrel, well I'd still probably go Whelen... But also the 7x57 is still popular enough that brass isn't too tough, and there are all kinds of bullets available at reasonable price. Also a great classic cartridge.