The only problem I would foresee would be if the lights you want to use have internal bulb shields. (Filament glare blocking shields) The HID element might reach in too far. Better to get a light that's sans-shields. If the reflector can be totally removed, (Not glue-bonded to the front glass) then you can cut the shields or bend them out of the way.
The only other thing that has to be done is find a way to get the large HID wire loom out of the housing. I just made a larger hole and silicone sealed it back up. The wire has to withstand the high voltage arc start-up, so it can't be swapped out for something else. The connectors on the ballast is also designed to contain that voltage. The HID elements run at around 85 volts after being lit up, but it's around 10,000 volts to get the arc started. You don't want THAT leaking out.