I'm getting a little out of my experience area now. I'm not familiar with the XP series dies so I don't know what kind of heat they generate. The XM series are pretty efficient so off hand I'd say the XP probably get very, very hot and would need finned or maybe a fanned cooling solution like you see in headlights:

Like these

SKU 9716188

I don't have a set handy but you might they might be able to fit through the existing bulb hole (they're right around 11mm in diameter). Since they are made for car size reflectors my guess would be you'd get an extremely bright flood light because they're pretty lengthy and extend well beyond the focal point of your existing reflector. A fog light is probably closer to your needs in terms of length:

SKU 6929402

But if you go that way you lose the ballast and heat sink you'd get in a headlight and be tied to a 3S battery.

Unless you're going for like 1000000000 candles or something, I'd go with a 16mm pill and XM-L2 emitter with a 3A driver. You might be able to shove that into the existing hole without having to do much modification (it's the more elegant solution and it leaves you open to other options if it doesn't perform to your satisfaction). You should have the mass and air circulation for passive cooling a 16mm setup as well. Then all you have to do is run a single (+) wire from the driver board to the battery.

Hope this helps. Like I say, this is about the limit of my experience doing LED mods. The only other suggestion I can offer is to keep in mind LED's are flat and don't emit over as many degrees as a filament light. This means getting a good beam difficult, but making a flood light with an LED super easy.

eta: and Wylie's link there to CPF is an excellent resource. you might find a thread on similar mods there