Using the stock pieces and only changing out the P60 lamp, Malkoff or Nailbender drop-ins made to fit a Surefire 6P will fit. Some people have reported them dropping right in with no issue, but I had to apply some judicious dremel work to get a Malkoff M61 to fit.
Malkoff also makes units that replace the entire head and body that screws into the forend.
The 6PX head is self contained and won't screw onto that forend.