I installed a 13 HP Predator (Harbor Freight Honda Clone) on my 1975 MF12G. I would have liked to keep the HH120, but it was cost prohibitive to find a piston and bore the block, much less the valve work and such. Anyway, it runs flawless, consumes about half the gas and is much quieter with less vibration. It wasn't a drop-in swap but at least the bolt holes in the chassis lined up perfectly. I know it doesn't relate to your Bolens but a re-power will have similar problems and advantages from brand to brand I would think.
I'm sure the shaft drives represent a whole lot of different problems as Eric was saying though.
Link to Predator engines
Folks on this forum gave a lot of great advice while I was doing the swap, especially Doug-DH1 who had done a couple of MF Hydro swaps before I did the gear drive swap.
Edited by MFDAC, October 22, 2016 - 08:38 PM.