Many thanks Mike for sharing your experience. That's worth much more than any secondhand info or guesses.
Thanks also for the offer bitz and pieces.
I've had the idea to upgrade to 1000 if anything terminal ever happened to the 900, for a long time, but I think you have confirmed to me that it's not just a simple two bolt engine swap.
I think I would go along with the idea of getting a M1000 or S2R rather than cobbling one up, and start fresh.
Besides dacs has offered me an old fridge motor... Sounds like the way forward..