funny that, i broke mine on my recent trip to the BMWST unrally in NV.
there are no replacements, its a proprietary design and all that...
i had the Reno dealer just order it in and swapped it in 15mins. there are 3 screws holding the seal on the backside of the cap. remove them, you will see the clip that holds the lock in place, slide it over the lock drops out (need to have the key in the lock), make sure you know the lock orientation when it come out so you can fit it into the new cap, the correct way.
unscrew 6 screws that hold the inlet tube in place install new assembly.
this is worthy of at least one ale...
failing all that go RED-Green .