So, I turned to good buddy Tim Ritchey, who had salvaged an identical General Electric 5BT1346B51 from a cancerous Jet Electric pickup, which looked good and "ran till they parked it", and it also spun smoothly on 12 Volts.
Pulling the Grumman motor was a bit challenging due to the tight clearance between the flywheel and the transmission output flange. VW's solution to this problem was to machine a scallop out of the side of the flywheel so that the engine could only be pulled out at one point in a rotation, and then only by snaking the engine around that output flange. It was quite the wrestling match with a 200 lb motor and flywheel. I imagine a big greasy engine would be much more fun indeed!
Once we got the motor out, the mysterious 'riveted on' brush covers slid right off after removing a draw bolt, so we could inspect the brushes and commutator bars. They are shiny copper instead of the expected dark patina, but with no visible defects, and the brushes appear to be in good order. To add insult, the rusty old motor spins just fine on 12 Volts.
Oh well, we have a replacement motor all painted up, and are going to test drive it, by gosh!
Pulling the 1980 Jet Electric motor hub was quite a challenge, apparent in the end due to the liberal amount of red Locktite that had been not only applied to the bolts, but also slathered all over the motor shaft! A weekend of penetrating oil and four guys on prybars finally popped it loose. Here's hoping that we didn't damage the motor bearings in the process, but they 'feel fine'.
The whole exercise will not be for naught, as the clutch was worn down to the rivets, the pressure plate was deeply grooved, and the flywheel was pitted from its years of immobile exposure. With all new/resurfaced components, we can at least eliminate a few more variables.
Just test-fitting components, obviously some learning left to go and not ready for installation yet!
We considered going clutchless, going as far as to procure a spare transaxle that was specifically modified for clutchless EV use, but the two provided adaptor hubs and transmission profile plate don't match my spare motors, so we'll hold off on any custom machining until after a test of the replacement GE motor with a new clutch.