Hello UWAK!
Sorry for not replying sooner but I havnt been on here untill today. I don´t remember exactly which dropin it was but any one that fits the Surefire 6p will work. Just pick one that you like considering output and modes.
I took of the reflector of the dropin and used the reflector from the ASP. I used superglue to glue the pill from the dropin to the reflector. I hade to use my dremel on the relector and in the inside of the ASP flashlight.
Mind you the light is now all flood but I like the format of the ASP and thought it was a shame to not try to fix it.
Good luck!