jimmy13 wrote:You could easily lose a kilo with a packet of exlax

Yes, but your exhaust valve would suffer!
You will gain ponies from having no load from the charging system. In simple terms, the energy that would normally go to your battery etc. would go straight onto the back wheel. Remove the magnets (like Matt said) and trim the flywheel in a lathe right up to the pickups. You could also shave off the thick steel. Or buy a total loss flywheel.
And then drink more beer