Well, to answer your question, I'd say dig that motor out....it should have a 1" dia output shaft, but I've seen them stepped down before too and if that's the case you might find yourself looking for a new engine. Also, most of us are running centrifugal clutches our racing mowers. If you are running a manual clutch, hopefully someone else can help you on the pulley sizes. I personally run 5" belt pullies on my clutches and have also tried 4-1/2" as well. Good luck with your build and ask as many questions as you need to because we're all here to help.