I have seen people use old generators from car engines for small electric go-carts. Not alternators, it has to be a generator. VW bugs had generators up till around 72 or 73 (not sure the year they swithed to alternators).
There are two things that make these work well:
1. the are designed for a 12V application
2. they have a separate field winding that you can use to vary the speed. The stronger the field, the slower the speed (with more torque)
also they have fan belt pulleys already on the shaft. I would take a wild guess that you can get maybe 5HP out of one of these!
just to give you an idea, this is what they look like: the ground is through the metal housing, one connection is the armature, the other is the field
http://cgi.ebay.com/ebaymotors/VW-TY... 4644044QQrdZ1