Hi Matt, i made up adaptors to be able to fit H4 headlamps in behind the glass on my 23. Not all LED headlamp bulbs are up to the task. Most are rubbish, I ended up getting 2 5000 lumin bulbs from Pau Goff in England. They work well, and because they are fitted into the correct lamps, they are legal.
http://www.norbsa02.freeuk.com/goffyWhyNotLEDs.htm
I ended up using the daylighter H4 bulb,
You need to choose a bulb designed for you headlamp unit. In my case, it was easy to fit the H4 units into my original head lamp housings, and behind the original glass