Re: Propane conversion
You will find about a 20% reduction in horsepower when converting from gasoline to propane. You can confirm that by checking the rated power output of portable generators that are offered with either fuel option. The propane fueled model will have a lower electrical power output.
For a propane conversion for small four stroke motors check into the conversion kits available for Honda EU series portable generators.
It would seem to me that a four stroke propane fueled bike would lose the whole idea of a simple cheap light weight power conversion for a bicycle.