Hardtails are usually the best setup for motorized bikes. This bike's rear subframe's pivot would mean that the engine would slacken the engine's drive chain under load, not a good situation when you want the chain to stay securely on the sprockets.
If Sick Bike Parts still made their Shifter kits, the pivot looks to be right behind the bottom bracket, doesn't look like a bike engine and shifter kit would fit unfortunately.