The main problem is a lack of availble power from the 'alternator', I've had various trail bikes and all have had the same type of electrics. There are usually two different alternator coils, the first is used to charge the battery, and side lights, indicators and brake light work off that battery. The second coil feeds just the headlight bulb, with the coil essentially being 'self regulating', this results in a dim light at low revs, with the headlight been reasonably good at decent revs.
However, the tiny battery, and only been a 6V system, plus miniscule power from the battery charging circuit, result in poor performance of the main electrical system at low revs as well!.
Unless your's is radically different to all the ones I've had?, reducing the load won't affect the headlight at all?, as it's off a completely seperate coil.
I gave much thought over the years to improving it, but never actually got around to doing anything! - I suspect the first thing needed is a better alternator system to start from? - and changing from 6v to 12V (with the subsequent halving of current and associated voltage drop) would be a good idea as well.