I've been tempted to experiment with some sort of LED headlight setup for my MB5 if and when the original bulb goes out. However I'm curious if there would be problems due to the wild fluctuation of voltage on these bike. There really isn't a regulator on the headlight circuit, and my concern would be the LEDs might totally cut off at idle and/or blow at full rpm. The original MB5 shop manual shows the headlight wattage at 30/31.5 so sticking with something close to 30 watts is probably best. That said I might try it some day.
Any luck with getting your bike to run wide open without cutting out? Cool air box mod by the way!