Hi
Nige, and a warm welcome to the forum. It's always nice to welcome another Scott Bonnar owner on board.

I presume you are choosing a new engine for your Scotty because your previous engine has failed...my question is this...was your engine a Briggs or a Kirby Lauson (Tecumseh)? This will give us an idea of the age of your machine, and give us the PTO diameter (ie. diameter of your engine shaft) of your previous engine that has to mate with the inner clutch half of the cutter clutch.
There is a difference between them; one is 5/8" and the other 3/4".
A horizontal shaft Briggs of 3 HP. would suffice, but you do have choices...I have re-engined my machine with a Honda G100 2.5HP and have found it faultless for my 17" Scotty.

Looking forward to your reply and once again,

to OutdoorKing.