Does anyone know if it could work? Would like the bigger motor in the smaller package. Don't want to just go buy a Rincon. Upon theoretical ponderance I see no reason it wouldn't work. Would the cylinder swap out??

 

The 420 engine/gearbox is a good one, the 680 is a Lemon of the brightest hue. The pre 2014 420 rear axle however (apart from the IRS version) can't take the power of the 420, never mind a 680 engine.

You may get a post 2012 500FM engine to go straight in, it looks very similar to the 420 motor, and I would bet the power output to the wheels will be close to that of the 680, but you would still have that rubbish rear axle.