We sourced a 1/2" bore 10 Tooth, 35 pitch sprocket from Martin Company to fit the pto shaft. Then we got the mini jackshaft and clutch mounted.
We're trying a nos 300 Series (3" diameter) Comet clutch I had kicking around because we felt the small diameter fit the proportions of the bike better. Since the jackshaft is only turning roughly 1/4 crankshaft speed (3.6:1) achieving enough revs at the jackshaft to engage the clutch is a bit of a concern at this point. I have an old dealer service kit for these clutches and we installed the lightest engagement spring available (1100-1500 RPM). That equals approx. 4,000 -5400 engine RPM. Mounting the clutch directly to the 1/2" engine pto is just not practical.