I think the stock flywheel/chassis hole is problematic, as I couldnt use a bump box on my S2... the wheel fouled the chassis without touching the flywheel...

I never bothered to check to see if it was the flywheel too small or the hole in the chassis, I sold it soon after to fund some other purchases...

If the shaft is still poking out the backplate when you remove the pull start housing, you may be able to stick a 6mm hex adapter on the shaft and use a hex start wand like the heli guys and rotostart guys use..