Just came across this thread and I am in the same boat - picked up an early Masport Home Gardener from facebook marketplace with a blown engine.
I have decided to fit a Baumr-AG 6.5hp from Edisons (as I can't find any 3.5hp units) at a cost of $121 delivered.
I will simply widen the axle by welding on some 15mm rod to suit some puncture proof wider wheels which will ride over the dirt easier (added bonus of having sealed bearings).
I'll post some pics once I get around to doing when I return from holidays.
