Actually it was the name of one of the earliest 2 stroke engine kits. The company that imported it to the states called their kit "Happy Time Bicycle Motor". The company, or at least their name for the engine is gone but Happy Time, or HT stuck and it's become synonymous with the Chinese 2 stroke, in-frame engine. They are also often refered to as China Girls.
You'll have to ask Dan how to mount one of those