Its not just the shaft diameter & length, it's also where the shaft end up in relation to the pulley system the engine drives.
You may be able to download the design/shape and measurements of the proposed Kwaka(s) or B&S upgrade, and use that to work out if the engine(s) will :
- Fit in the available space (remeber, its not just fitting, you need to be able to mainatine the engine , change oil, filters, spark plugs, etc)
- PTO shaft will end up in a very similar position to the OM engine
- What modifications may be required eg drilling new holes in the mounting plate and even welding on additional plate to match up with a new hole position.
Most of the other "stuff" fuel line, controle cable(s) electrics, can be lengthened / shortened, rerouted, etc.
I have done this on a Hustler - Went for a shade more hp - new engine came with its own muffler - it all worked out but its fiddley, not as straight forward, as you may imagine.