I just finished my M20 flywheel swap. My flywheel ended up being from an M20 Super Eta. Basically, as long as the single-mass flywheel will work with an M20 engine it should work just fine on the M42. They all have the same allignment of the ring gear/starter pinion, and that is all that really matters.
It's also been verified that M40 (Euro-spec) flywheels work too. Just to be safe, stick with only M20 single-mass flywheels.