Do you have a link, schematic, even pictures of what you want to build? I've got a 6.5 Hp Honda lawnmower engine and several spare-part alternators from various vehicles I use to own, the idea is interesting. Been wanting a welder, but not enough to shell out the bucks. I've had a few project ideas lately that need some light welding, usually get my little brother to do it, but would like to be able to fire it up when I get the urge to build.
About all I know about alternators is that the rectifier is built in (usually), and that after your car is running, you can disconnect the battery, and it should continue to run, unless the alternator is bad.