It will take more than just resistors. You will need to get the voltage regulated somehow (zener diodes) or you could easily end up frying your wiring harness. You cannot just load it down, the power has to be disapated somewhere, think Prince of Darkness. If you just put the excess load onto resistors, they WILL get very hot and very likely end up melting all your wiring.
Take Sandman's advice, get a LED regulator kit and relays. If you don't know how to do this stuff, it is better left to those that do. You could even possibly end up with the bike catching fire if yiou mess up electrics. Hmm, wiring burning underneath the gastank....not a good mental picture.
