I have an old Toro Whirlwind mower that has the 9hp model 23 engine. I think mine is the AB model. I rebuilt the engine about 20 years ago.
The block was bored oversize and the crankshaft needed to be ground undersize. It's a pain to start at times has a lot of compression with a rope start. I belt start it off my garden tractor at times using a v belt. When I had pole lima beans that mower made short work of the vines. Hal