If you want to keep the "original" generator-look, any local auto electric store can re-wire the generator for 12 volts. It'll cost you around $100.00 (an alternator can be had for $35 to $50 depending) and it will have a different regulator mounted on the generator - one of the square, Delco-Remy regulators used on may GM cars over the years, so the only visual difference will be the relay.
If you go with the alternator get a 5/8" belt pulley for it from Genesee Products to match your belt - they also have wiring harnesses, etc. Nice folks to deal with (link below).
I've seen both and both have their strong points. Do whichever suits you best.