First, yes, you can still pedal as a normal bike with an engine kit fitted, but with a HT kit there is more drag due to the extra chain and clutch friction.
Usually, it doesn't matter what gears your bike has, for a standard single-speed setup.
Having said that, a Sick Bike Parts
shift-kit transfers a HT engine's drive from LHS to RHS, into your normal drive-train, allowing you to use your gears.
I have one fitted, driving my 6-speed derailleurs. I'd never go back to a single-speed MB.
Can get down to 5mph in first, or 50mph in 6th on a slight downhill.