Easiest thing would be to set up a jackshaft primary drive and use a pully on the engine instead of a centrifugal clutch, and fab a spring loaded idler to press the outside of the slack side of the belt, with a cable and clutch lever to control the idler to disengage the belt. Instead of jackshaft, some have used a bike rim attached to the rear rim for a pully to get a useable ratio, and modded the frame for it.
Search the DIY section . Look at my in-frame Preddy friction drive thread, And Cannonball2 Just posted pics yesterday of his 1905 replica with a belt clutch and Robin engine.
Well that can be done. I have pop cans I can melt and as far as the witchcraft goes I have an ex-wife so that probably has that covered... Thanks Charlie!