Motor for Avon R310
Do you mean the Rover 310 or the Rib 310?
My first 1979 Rover 310 would plane on a Tohatsu 3.6. They haven't made that engine for years, but it might give some idea. By the way, I weighed 10 stone then.
30 years later, I had an early rib 310 which planed happily with a Yamaha 5. Now I have a 311 RIB, and it would just plane on the Yammie. ( I now weigh 11 1/2 stone). It now has a Tohatsu 9.8 2 stroke on and it flies with Wendy and me in it. Our total weight is 20 1/2 stone.
All the Avons of that size use standard shaft motors.