If the existing fuse is rated at 10 amps then the wiring should be good for 100W @ 12 volts, although I believe 55w is the maximum for H4/H2 bulbs for road use - check out the Halfords Rally bulb range
Disclaimer - the above is simply based on Wattsits (Ohms) Law 100 (watts) / 12 (volts) = < 10 (amps)
Assuming you have a 10 amp fuse for the lighting circuit then the wiring should be good for 120 Watts. (Good old watsits law again.) But that’s the total load, so you have to deduct anything else that’s running off the same circuit. Commonly that is rear light, brake light, instrument lights and sometimes indicators.
Apart from the rear/brake light, most is pretty trivial, so another 10 watts or so at the headlight should be an easy doing.
Like Kevsta said, Philips Brilliance xtreme, and 45/45W should be no problem.