Re: heavy riders
As has been mentioned, the wheels are the key thing here. I'm pretty close to your...err...dimensions and the biggest modifications I've worried about are the wheels. My "safest" bike has thick spokes, 12 or 11g, I forget. ALL my bikes drive through the hub, get rid of the rag joint, it's a spoke killer.
Beyond that just be sure you check it over OFTEN for wear or loose parts and carry a tool kit.
Technically, I should point out that most states group motorbicyles under the "moped" category and you generally still need a valid license to ride them.