He uses:
Rickenbacker 4004LK (favor neck pickup and turn tone full) --> Marshall JMP Superbass II (cut bass allmost of, cut highs slightly and boost mids. Precence and volume up to 3`a clock) --> Marshall 4x12 and Marshall 4x15 (custom)
I think you can get close with some Rick and Marshall guitar stack..
I gues it won`t sound really good if you play something else than just Motörhead...
Some one here at TB must have tried to copy him, so he can tell you more...
edit. ah I was too slow..
I think you can get some what Lemmy like sound with overdrive/distortion pedal, mid boost eq and use pick... (I get it pretty close with my jazz bass..)