I don't think there are any US made bike engines, but Grubee and BikeBerry.com are well known US based importers that sell the 66cc chinese motor kits.
The stock china-kit in-frame motors are ok, but the stock idler wheel and sprocket adapter (rag joint) leave a lot to be desired.
Check out bicycle engine kit, bike engine, bicycle engine, bicycle motor
for friction drive and chain drive rear or front rack systems that take a variety of engines (pull start, look like big weeder engines).
Dax also has the in-frame engines.
Bikeberry.com has the in-frame engines, and sells 36 tooth cogs and metal "pineapple sprocket adapters" which will simplify your build a LOT and make starting easier and top speed faster. Good luck and welcome to motorbicycling!