If you are just tinkering then any bike can usually be converted. If you start with a junk bike, you end up with motorized junk. You can always change the bike out latter after a learning curve.
I personally would never even consider a gasoline ICE setup (internal combustion engine). The power curves for an ICE are backwards from an electric setup. With an electric setup, maximum torque is at zero speed. On an ICE maximum torque is somewhere higher than 50% output. Torque is important for climbing and stop/start/low speed work. The ICE/gear changing setup is very difficult to integrate smoothly and always results in some serious compromises unless you are very good with designing and building electronic integration setups. Geared hubs are better but still compromised. . ICE engines are also not included in bicycle laws in most jurisdictions and will not pass muster on anything but the roadways..