That's a very cool bike! Its just what we would have built if we were back in those times. In fact it basically the same as my last build though much cooler being vintage of course. http://motorbicycling.com/showpost.p...6&postcount=14
I am building a diesel powered fishing boat that I want to use a cent clutch on. I want it to engage around 1200/1300rpm. I found at the Maxtorque site they have springs to do just that. Used with their pulley clutch it might be what you need.