I believe the real solution will be accomplished by the Chinese. One of these days they will buy an old vintage HD motor or something similar and clones it it fit a bicycle. A $$$$$ investment in an old engine, another big chunk of money for making mass produced molds. They will probably use the current Chinese engine parts like the connecting rids, the pistons rings etc.. Interchangeable in parts, less cost in inventory and manufacturing.
First they have to see that the demand is high enough, the downside, the engines will be too large for most bicycles, and increases the speed past the legal limit for most states motorized bicycle laws. This will put them in the motorcycle category.