This is the problem with a lot of the toro professional stuff - parts are hard to get and cost a fortune.
I have only ever seen one of these and from memory there is a cog that is either cast on the end of the crankshaft or is part of a sleeve that fits on the end of the crankshaft.
I don't see why a honda gx120 or 160 wouldn't work provided it all clears and the cog is on a sleeve. It should just be a 5/8 or 3/4 inch shaft, potentially (but not likely) with a reduction box.
If the gear is integral to the crank, then you have problems
