[John316]

I recently picked up a Yerf dog spiderbox from a friend. It is in bad shape. Carb was seized up, engine seized up. I drained about 1/2 gallon of water out when I drained the oil. I pulled apart the head to look at cylinder, but decided the engine wasn't worth salvaging. I looked at all the available options for a new engine and choose to go with a 13 hp predator with 40 series torque converter, jackshaft mounted on pillow block bearings. I already have the engine and I'm welding on the frame to mount it.
The only issue I still haven't figured out is what to do on the sprocket mounted on the rear axle?
It appears to be a 31 tooth for a #530 chain. I want to go with a #35 or #420 chain and sprocket set up because of availability and variety. (I was thinking of going with a 12 tooth drive sprocket and a 60 tooth driven). I wanted to keep max speed around 35-40 mph.
I am having a hard time finding a 60T sprocket with 3-1/8 bolt hole cir (2-1/4 hole to hole) bolt pattern.

Anybody know where I can get a 60 T replacement sprocket in a #35 or #420 ?
Or any other Ideas that might help. Is my calculations on the gears seem close to the MPH I am looking for?