It really helps to show your exact bike pics or a link to it on-line.
There are electric options that can fit mountain bikes with no center cavity for gas, and let you use your bike gears with electric motor power.
That full suspension sucker packed about a KiloWatt of power and was not just a rocket off the line, it would do over 40MPH thanks to the gears and sheer power.
Not cheap like your bike I would assume or you would just buy a decent beach cruiser bike to center mount a gas engine on.
Beats the hell out of some rinkey dink small gas engine stuck on top or to side of your nice bikes back wheel.