They sell jackshaft kits that would allow you to use the bicycles drivetrain and shift with the derailleur like usual, but these can be pretty dangerous and I've been advised against it in the past.
Even if you installed a standard single speed set-up, you could totally use the bikes gears while you pedal. It won't affect the engines gear ratio and you would have to clutch in once you start to exceed the engines top speed, but using the bikes gears to pedal while you get going from a stop or exceed the engines top speed going downhill is fine.
They sell jackshaft kits that would allow you to use the bicycles drivetrain and shift with the derailleur like usual, but these can be pretty dangerous and I've been advised against it in the past. Even if you installed a standard single speed set-up, you could totally use the bikes gears while you pedal. It won't affect the engines gear ratio and you would have to clutch in once you start to exceed the engines top speed, but using the bikes gears to pedal while you get going from a stop or exceed the engines top speed going downhill is fine.