It's doubtful you're going to get it out of there without damaging something. I'd wager that it is held in with silicone. If you can see both, the output coupler and the high reflector, you should be able to determine the tube length. Once you know that, the power supply shouldn't be too big a mystery. HeNe's don't need precise voltage/current. You just don't want to feed it too much because your output will be lower and the tube lifespan will be shorter. Judging by the length you show, I would guess about 1800~2300VDC @ 5.5mA
If you're the smartest person in the room, then you're in the wrong room.