Howdy Tray. Welcome. Is a bit of a broad question and there are many choices and upgrade options available and in prototype. That being said, I would contact Duane at thatsdax bicycle engine kit, bike engine, bicycle engine, bicycle motor
303-459-2780. Good guy and really knows MBs. Also Bicycle Engines, Motorized Bicycles, Bicycle Motors, Motored Bikes: 2-Stroke & 4-Stroke Kits!
is a fine outfit that offers grubee. and Spooky Tooth Cycles - Motorized Bicycles: Gas Powered and Electric Bikes - Home
There are many others, but I can say from personal experience, these are good folks. Hope that helps and good luck with your endeavor.