There have been a few recent cases in the news about accidents involving bicycles that have been retrofitted with gas engines. These engine kits are widely available and many people think it is a legal thing to do the conversions.
It is NOT legal to convert your bike to a gas powered bike. Once you do so, you are no longer considered a bike, but a motorcycle. You would need a motorcycle licence, insurance, and a helmet if the vehicle passes a safety inspection (which it won't).
The speeds at which the bike will travel make them entirely unsafe for use on public roads, and we’ve seen the results in a few accidents already. Bikes are not designed to travel safely at speed, nor to stop or avoid obstacles at speed. The danger is not only to the rider, but to any other pedestrian or cyclist hit by accident.
If you have converted your bike already then understand you cannot ride it off your own property.