Hey Chip,
I had a '79 XL250 for years. Loved the bike except the pathetic front brake. Did a big bore kit and ported head (both Honda factory accessories!), as well as a round slide Mikuni. Tons of fun!
As for the pathetic electrics, someone turned me on to a funny idea. The bike basically had three electrical systems. Part of the magneto went directly to the ignition. Another part went to the lighting. Funny thing was that the alternator didn't know if it was 6 or 12 volt. All I had to do was run a simple 12V voltage regulator for the headlight, and Voila! 12V quartz Halogen headlight. So, the headlight was running on AC current.
The bike never had a battery (PO removed it and the battery box). I think I ran a one piece regulator/rectifier to power the tail light and idiot lights. So that part of the bike ran on 12V DC.
The reason for all that weirdness was that some law in some country specified that a tail light had to be on even if the engine wasn't running. Hence the tail light being tied into the battery/charging system. The headlight only worked when the bike was running.
These are 38 year old memories, but I'm pretty sure it's all correct.
Look at the wiring diagram and see if the headlight comes directly off the battery. And if there is a regulator in the system.
If the headlight is running off the alternator with no regulator, remove the headlight and put an AC voltmeter to where the HL connector. Rev up the bike and check the voltage. Should be a lot of volts in a hurry. The headlight was actually the load resistor.
Not sure if this helps, but having a real headlight on a bike is a nice thing