id be more inclined to believe he roached a sprag.
and iirc he went to a stock converter so it could be the turbine sprag is shot so it could just be a converter.
then again burnouts of fat rubber would probably shock any number of things.
do a quick test. start the rear and immediately put it into gear within 1 second of startup. if you roll a few feet then loose all oomph, it probably the turbine sprag.
ive dealt with two of em. both times the clue was that it would only move for a short time immediately after startup before the turbine could start slipping/spinning
iss speed would rise and fall with the acceleration but stay at 0 (the driven systems functioning, chain stationary, turbine shaft not turning....
id half venture it's probably worth the inefficiencies on the back half to just do a spragless hub (weld it) and see how that goes...you be able to pedal it in/out of power without shocking the sprag by making it lock/unlock/lock doing so.
what do your logs show for the ISS/crank rpm at the break?