Someone over on my-mc tested a new 40Amp alternator, and it actually makes over 50 Amps. The 40Amp unit can be rebuilt to some degree, but you have to know someone who really knows what they are doing - hard to find these days.
As long as the windings are OK, you can rebuild the 40 amp yourself. I did a writeup on this when I rebuilt mine a while back. See it on the "other" site: HERE