Hi MickyD,
Did you measure the compression, pull it over a few times? I'm wondering if their isn't a leak somewhere or if something is physically stopping the engine from turning over properly? These engines left one of the valves ever so slightly open to help it make it easier to pull start, did they not? Perhaps something is not quite right there? Other than that, I can't think of anything else right now, seems like a tricky one.
