From previous threads, I gather the earliest HD8 mowers had a rather inglorious Fuji (Subaru) Robin engine, which was followed after a year or two by a side valve Honda - which may or may not initially have been optional. Possibly this machine started out with a Fuji and it was ditched in the interests of improved behaviour and less overheating, or it may have had a G65 Honda to start with. The GX240 currently in it, should be an improvement on either. However for such a rare old machine, the original engine is to be preferred, or at least kept locked away and shown to people who stop by to admire the machine.
For some information on early HD8s, check this thread:
https://www.outdoorking-forum.com.au/forum/ubbthreads.php?ubb=showflat&Number=40749&page=1
