Remember K.I.S.S.? You fellers are thinking way to hard! I've built 3 happy Time kits. Scratch built 2 rear rack V belt drives and 2 front wheel drive V belt drive motorized bikes. V belt drive is with out a doubt the most trouble free way, chains are dirty, greasy and drag like hell.
What you do for the giant pulley/sheave on driven wheel is take the rim/ hoop of a smaller doner bike and zip tie it using hoops spoke holes to the spokes of the bigger build bike wheel ie. (26" hoop / 700C wheel) (24" hoop / 26" wheel). The large hoop/pulley with a 1.5" pulley on motor shaft will give you about a 30 mph top speed.