Hi
I have used that valve in a custom airsoft gun.
The diameter is wrong in the picture it is 1.5 inches (34mm) in diameter.
If you have access to a lathe you can dismantle it and carefully turn it down to fit a gearbox.
I am still working out how to connect a 90 degree bend to the inlet so as to keep the hose inside the gearbox.
Will let you know how I get on