You may have to verify this, but I think that light has a halogen capsule mounted onto an automotive bulb base, and for some reason, H4 seems to be the most common automotive bulb base used in these spotlights. After double-checking what bulb you have, you just get an HID kit with the same automotive bulb base, and I think that's all you gotta do. H4 is weird, though, because the H4 automotive bulb is a dual filament, while you will want to find a single HID capsule on an H4 base. You may have to make some spacers to get the arc into the focal point of the reflector because HID kits are generally not the greatest optical quality in terms of precision of placement.
DDM has H4 singe mode HID sets available cheaply. For around $40 you get a kit with 2 bulbs and 2 ballasts. They do ship from overseas so shipping runs around $20 and can take 2-3 weeks