I'm assuming you're using a double walled rim. I personally prefer steel rims over aluminum alloy rims.
You may want to consider Staton Inc hub. It isn't a flip-flop where both sides are right-hand threaded. The SI hub has right-hand threads on the right hand-hand side and left-hand threads on the left-hand side. He also sells adaptors to mount sprockets to the hub. The jackshaft disc brake combined with a rear rim brake can be another option if your idea seems to have problems. If you consider the SI hub option read an old thread I did on it. I still have that wheel.
A few months ago the rear axle broke in my Staton Inc hub. It broke where the snap ring groove is. So I had David redesign me a new axle. The new axle no longer has snap ring grooves. Instead it now uses sleeves to hold the sealed bearings in place.
motoredbikes.com
I will give this advice on the legal side. If you want to pass as a motorized bicycle stay closer to a bicycle look than a motorcycle look. It can be hard to convince some police that your vehicle is a motorized bicycle when it looks like a motorcycle/motor-driven cycle. Just because you're a traffic law abiding rider doesn't mean some situations may cause you to have an encounter with the police.
If you're completely meeting the legal guidelines of a motorized bicycle; make it look as much like a motorcycle as you want/can.
If you're going outside the legal guidelines then the last thing you want is something being presented as a motorized bicycle being perceived as what it actually is; a motorcycle/motor-driven cycle. It won't go well if it's discovered your bike doesn't match up to its paperwork. Without looking them up, I'm not sure if your state will allow a self-built motor-driven cycle.
In my state Tennessee, a shifter bike isn't legal, it legal statute has an automatic transmission clause. This was why I built the LandRider which shifted gears automatically. I now ride the Sidewinder which is a manual shifter. The police have never bothered me. My bike can't go faster than 30 mph on level ground. My bike looks like a bicycle. If my bike looked more like a motorcycle and I was doing more than 30 mph on level ground; they'd have a real problem with the shifting clause.