I've been doing a bit of modification on my 15 year old home made personal recumbent trike again. This was one of the first trikes I originally built. Since then I've built and sold many of them locally to people.
The rear hub motor was removed and a 49cc 4stroke engine was added to replace it. This motor has a built in jackshaft so that made things a bit simpler when installing it.
The trike has about the same top speed ( 30 mph) but now I have a lot more range with it again. The little gas tank holds about 2 quarts or so and that gives me about 40 miles of range. It will get me all the way across town and back before I need to add more gas. I also use straight gasoline in it - no gasahol! That stuff ruins small engines.
This is the third gas engine that has been mounted on it in the last 15 years. It's also had 4 different electric motor setups used on it. It's basically my "test it out" trike that I use for testing various assist motor/engine setups.