Man, That's a great idea!. Can't wait to see what you come up with. The Preddy's look so right with the cylinder facing foreward. Makes the intake and exhaust easier to deal with too.
I have a couple nice suspension bikes and couple more small 4 strokes that could be adapted to this type mount. Have you seen my in frame preddy fricton drive thread, and Cannball2's multiple threads on jackshaft driven fricton drives. Plenty good info about roller sizes, ratios ect.
One thing I would recomend right off is to use a belt drive on the jackshaft. The chain drives are problematic. The fast moving chain is noisy and wears quickly and needs plenty maintenance.
From my experience,the belt is quiet and smooth, hardly ever needs attention, long lasting, delivers more than enough torque to drive a roller. The roller will slip before the belt.
I would also spend the few extra bux for the Maxtorque clutch with the sealed bearings instead of the bushing. The bushing clutches are high maintenance more noisy.
Another thing that helped my FWD bike not fall over so easily from the kickstand was to add an axle mount kickstand to the front axle.
Good luck with your build.