I just put an X-Alpha drop-in in a 3P and it will run on a primary cell. It has 5 modes in it. However it is much brighter with a 3.7 Li-ion cell and that's what I use. I have not tried it with the stock bezel, I have a 6P bezel on it. This will leave you with a slight gap between the bezel and the body. I used a GID o-ring or a black one will work too and it fills the gap perfectly.
You can use the stock twisty to change modes or you can use a Z59 tail
which is what I use. This will allow you to change modes VERY quickly. The beam on it is very nice, will easily light up a small room. Right now I'm having Milky pump up a drop-in for a 3P. In case you hadn't noticed, I'm a big 3P fan. Have fun with this light, there are quite a few combinations you can put together. Now if they only made a 7.4 volt CR123........If you want some pics, send me your Email addy.