Been reading through this thread as I picked up an ASG MP9 a while back. Since most of the activity here was back in 2012 I thought it may be useful to write about some new parts which have become available which may be of interest to more recent readers of this thread.
First up G&G have produced a bayonet to 14mm CCW adaptor to fit other silencers to the ASG/KSC/KWA MP9/MP9R without having to replace the original silencer mount. I wont post links to sites as so many in this thread are now 404. Please use your search engine of choice and vendors should become apparent....
G&G 14mm Negative Threaded Adapter for KWA KMP9 Series Airsoft GBB SMG's
RA-Tech have produced a new NS2/S7 compatible hop unit which is available with a hop rubber and 6.01 barrel
RA-Tech ESD EVO II Hopup Set with Precision Inner Barrel for KWA MP9 Series Airsoft GBB SMG
Wii Tech have brought out an upgraded hardened steel disconnector which may fix one of the biggest (from later thread posts) failures.
Wii Tech CNC Hardened Steel Disconnector For KSC / KWA MP9 Airsoft GBB SMGs Part no 153
Wii Tech CNC Hardened Steel Sear For KSC / KWA MP9 Airsoft GBB SMGs Part no 151
I gather if you have a nozzle failure with an earlier model nozzle, before the redesign, and need to replace the nozzle due to failure a new KWA/KSC replacement will also need the new orange piston seal rather than the green seal you will have fitted with an older nozzle.
While checking out mine I noticed a lot of BB fragments / dust around the hop unit. On closer inspection the Feed Ramp Base (part 18) had been damaged on the feed ramp fingers/prongs. The tip of one had cracked off. Both had nicks and gouges on the leading edge, where the BB first impacts them when stripped from the mag by the nozzle, and had become quite sharp. They were chewing chunks out of BBs so looks like I need a new one. This part is steel already so may be worth keeping an eye on for damage if you see lots of tiny BB chips around the hop unit.
Lastly while my bolt had no signs of wear from the bolt stop I did see signs of similar wear in the rhs bolt guide channel. As the bolt guides (part 66) in the lower receiver are pressed / stamped steel sheet, as with the bolt stop, it isn't too surprising to see that sharp edges / burrs of the pressing affect the bolt in the same way. A spot of delicate needle file fettling has removed the burrs and will hopefully eliminate the bolt wear.
Hope this is useful.