I too live in Texas, down in the Brownwood area, the cops here are very cool with my bikes so far, they just look and sometimes even wave or give me a thumbs up with a smile, this has been nice so far.
Now to the "stealth" you mentioned.......that is gonna be a hard one to accomplish with a 20" bike of that style frame, I dont know of any engine that will fit inside the frame, they are just too small, I also plan to build up a 20" in the future but I plan on going with either an engine mounted over the rear tire or most likely I'll build a pusher trailer, there are a couple other options that can be done but either of these will work the best IMHO.
A simple pusher set up with the HF 2.5HP might be a very good way to go, if you dont mind the tag along set up of the trailer, or just build you an engine mount behind the seat and put a 28T -30T Sprocket on the rear wheel and you should have a nice 30+ MPH cruising 20" bike, that little 2.5HP 4 stroke engine is gonna be about as quite and stealthy as it gets.
The only way I can see you mounting an engine in frame would be to cut the frames lower tube and then fabricate another tube that angles more straight forward from the cranks and then sharply angles back up to the steering neck, this would not be that hard to do if you have the tools for it, this would be a very cool set up but the bike would be severely alterd, but in a good way if you really want an in frame engine on it.
To keep the bike in it original form, I say go with the pusher and Disguise it as a cool looking bike trailer, get a 16" rear wheel from a kids bike and put a 60T sprocket on it with a rag joint and then drive it with the 2.5HP HF engine with a centrifical clutch and a #41 chain, this might make a neat little set up that could be made to look like a pull behind trailer and could actually be used for toting a few lighter things around like a 6 pack of "bokes or ceer" in a small cooler........!
Just some of my thinking on this, I hope whatever you do works out great..... Good Luck
here is a link to pushers that could be improved on and also a link to a very fast set up too.
YouTube - Gas powered Bike Trailer
YouTube - engine powered bike trailer = moped
YouTube - Leo83bmx's Channel
YouTube - ELECTRIC STARTER PUSHER BIKE