Steahly units are bolt on. I also doubt you'll be able to get one for the bike. However, if you befriend someone at a machine shop with the ability to balance a small flywheel, you could just have him tack weld a weighted ring onto the flywheel. I emphasize tack weld because if you get the wheel too hot, it will lose its magnetism. You could also send the cylinder out to be ported to tame down the hit and just not use the flywheel weight.
Any machine shop will be able to make and weld the ring for you. An electric motor shop should be able to balance the unit for you.