I decided I'd have one more go at this... I think it worked out ok. Found a non running mower with a Quantum XE 45, 15 minutes away, for $35 . They said it ran only months before, it was full of oil, not seized, had compression, and unlike most sellers, they were happy to give me the part number of the motor so I could look up the crank specs before going to pick it up. I did a comp test (90psi), cleaned out the carby, and it fired up and ran perfectly, just has a leaking needle and seat.
Modified the mulcher so the motor would sit at the right height, and it all mounts up OK. Surprisingly, the fuel tank even clears the chute - I was pretty certain I'd have to make it sit on an angle to clear, that would definitely have been needed with the intek. Anyway, it seems to work OK. Maybe a bit down on grunt compared to the original motor - I never used it - but it's working for what I need it for, and it's cost very little. Might modify the starter so the rope comes out to the side.
As for the 6.75hp Intek - I ended up mounting it in the chassis the Quantum came in, and attaching the weight/blade to the crank. It definitely cranks far easier with the inertia from that. It still misfires a bit while running, and blows smoke, but given what I've seen lately, it's probably a blown head gasket rather than rings. I'll pull the head and pan off sometime to check the condition of it all and decide whether or not it's worth putting new gaskets in.
