First order of business is to check what kind of voltages you're working with. I don't think the alternator puts out that much jam at idle, but if you're not over 12V at idle then your battery might be the issue. The next thing to check is the voltage when you rev the bike up. If the voltage goes up to around 14.3V-14.5V when you rev it, then the generator is working just fine. I'm not sure how one would check the voltage regulator effectively....maybe someone else can chime in on that. The regulator changes the voltage to the coil at the stator which then changes the voltage that's created by the windings. What year is your bike so we know if you have a mechanical or solid-state voltage regulator?
The problem may be in your bulb (they dim over time) or it might be your wiring (connections corrode over time increasing resistance and lowering available power for the headlight.) You can check the wiring for resistance with a meter as well..... you should be looking for as close to zero Ohms as possible. (single digits is best.....but if you're in the double digits you might want to clean up some connections).
LEDs will certainly lower the draw on the system which could help, but you need to get to the root of the problem so you can fix it.