Engine runs fine but seems to seize after wot

How did you test for spark? Pulling a plug and watching the spark jump the plug gap is not nearly an adequate test; a weak spark can jump the spark plug gap in open air but not come close to jumping the gap in a compressed fuel/air mixture. In open air, the spark should jump a 1/4" gap or more. ANd I assume you are testing spark whiel cranking? Please confirm.

Starter not turning fast enough would be critical for a diesel but no so much for a gas engine; don't focus on that, IMO. Either the spark timing is going waaaaay off for some reason (but does not seem likely if the engine runs OK otherwise), or more likley you have a fuel problem: either flooded or no fuel.

So have you determined for sure that the engine is not physically seizing/locking up? The symptom descriptions seem to be all over the place, so it would be helpful to everyone to make sure what is going on; we can't be with you in the garage to see directly and pick up clues.