the academy L85 can take any aftermarket AEG piston, well its worked with all i've tried, i gutted out my old TM M16 and put the stuff - all the gears (stock TM), spring (M100), piston and piston head (guarder), nozzle into my sa80. works fine.
in relations to points raised earlier in this thread, both M16 and Famas hop-up units will work, though with the M16 hop-up unit you will need to make an extension for it, to aid feeding from the magazine, on the Famas hop-up this is not needed, so for a no hassle option the Famas unit is easier to install and get working.