After a bit of searching I came across this:
WE G18C Broken Loading Nozzle Replacement Theory? (HD) - YouTube
He uses uses an AIP G17 nozzle in his WE 18c, after slightly modifying the nozzle, and (I believe) using a stock piston head and o ring.
He has a couple videos that you could look through for ideas/ solutions to the WE loading nozzle/return spring problem.