Hey Bonnar Nightmare, and a big warm welcome to the forum.
I had one of these machines and may I say they are very hard to get parts for.They were known at the time as a "Terrace Mower"
Turf Machinery Services may be able to help you out with the clutch parts you need to restore the old girl, as you pointed out, the gearing is critical as it governs the walking pace of the machine whilst cutting.
I have a Honda powered Scotty Model 45, and you can see the pics in the "Old Mower Pictures","Restored Model 45" thread on this forum if you are interested. I have used a 2.5hp Honda 4 stroke as it was the easiest to mount without having to reshape and modify the lower handle assembly.
A 5 hp Honda on a Model 40 would be a tight fit; considering the height of the horizontal engine shaft I doubt if it would line up and most probably be to high to suit.
Perhaps another option maybe to purchase the one from Turf Machinery Services ($220.00) which is all complete and running and then restore one to mint condition, using one for parts.
Hoping this has been of help,
