I've just refurbed a classic army spc, I do have a spare solid stock if your after a replacement for a few quid.
I did have difficulty wiring it to a sliding stock as the small tamiya connector was in a tricky place, I ended up having to chop it off and add some more cable via some joints in the gearbox.
Mine had metal bushings, the bits I changed were the piston and piston head (the one on mine was terrible), the air nozzle (be careful as the m4 one I bought was longer and caused feed issues), spring guide (as the one included looked low quality) and the tappet plate.
I'm going to probably change the spring, cylinder and cylinder head next, but want to chrono it first at my local site.
Good luck, swapping the solid stock to a sliding variant was very frustrating due to the highway pokery to get the cabling through.