Depending on the rear wheel size, mount a sprocket on the peddle side, or use a 12 inch rim with a bmx freewheel sprocket. Bonus for a bike frame that accepts a scooter rim.
Make a swingarm to hold an IGH so that you can adjust chain tension to the rear rim. Line up the output sprocket with the rear rime sprocket.
Line up the engine sprocket with the IGH input. Order different sprockets to get the right ratio.
If you must have peddles in your area, work on that aspect. Will the cops check for operational peddles and bump starting?