The cap checked out. I did the VOM test and then did a 350VDC leakage test. It's capacitance is 38.4uF, well within tolerance. The rotor, exciter and stator all seem to ohm out OK. It's a little hard to tell for sure, as the resistance is low and trying to get a good connection isn't easy. However, the resistances are very close and if they were too low I'd expect to see some burnt windings.
The rectifier passes a low voltage VOM test. Since it is a 1200V, 25A diode, I decided to try a higher voltage test on it. At 400VDC it will pass the full voltage one way but still allows 35V to pass the other way. I had a 1000V, 12A diode laying around and it only passed 5V backwards. So, since everything else seemed OK I found a diode at Mouser that should work, 1/2 the price of the dealer one. What do you guys think, is the diode bad?