I've had two different brushless motors on a Leimbach pump. Initially I used
This discontinued motor which spun pretty quick, but has a standard 1/8" shaft and mounting holes like any other 540/550 motor (25mm I think). All I had to do was cut the shaft shorter.
That was overkill so I tried out a tiny outrunner which I got from china. It needed an adapter plate to mount it and I a machined a new coupler to work with this motors smaller shaft (2mm). It has no problem running the cylinders.
I looked at the motor's Kv (which is rpm per volt applied) while considering my battery of choice. If the mounting holes and shaft size match you'll be good to go. If you want to match the max rpm of the leimbach system (12000rpm) you'll need a motor with around 1000Kv for 3s lipo, or 1600Kv with a 2s lipo.
Don't mind the sideways pump, my lines are all tangled so I couldn't reinstall it rightside up.
Lmackattach: I began looking at brushless only to decrease the footprint of the pump.