Just a normal H4 ( twin filament ) headlight bulb, many +120%, +150% and even +200% available from Philips, Osram etc.
It is a pain to get to - take off the wheel and pull the plastic mudguard out out the way - when replacing the bulb DO NOT TOUCH the glass ( if you do wipe it thoroughly with methylated spirits, if you leave any sweat or oil off your skin on the glass the glass will fail very quickly when it gets hot during use ). When replacing the bulb it helps to remember that when looking at the 3 projections sticking out of sides of base (not the electrical plug ) the widest one is vertical when bulb is being fitted into base ( also the widest prong of electrical plug will be vertical and at the top ).