It doesn't have to be too expensive, here is a good info site:
1 Wire Alternator Tech Page
Common Repair parts kit:
SE590 Self Exciting (One Wire) Conversion & Repair Kit for 10Si Series Alternators
If you have a Ford, Jegs has new replacement 1 wire alternators for less than $100.
JEGS Performance Products 10127 JEGS 1-Wire Ford Alternators
If you have a Chevy those are even easier.
Good luck, I used to rebuild those all the time back in the 70s and 80s, not hard at all.