To mount a Mark II side panel on a Mark III, I would replace the battery tray to a Mark II or earlier as well. That way you can retain your tool box and the battery tray also has the lower rear side panel mount already on it. Another thing worth noting is that the earlier side panel has a cutout in its leading edge for the ignition switch which the Mark III does not require.
I had a forum member contact me with a problem in that the starter motor's battery terminal interfered with the ham can style air filter. He ended up getting a narrower air filter from RGM motors and I machined a couple of carburetor adapters for him. Unfortunately he never contacted me to inform me if it worked out. I don't know much about the 850 Mark III's (I have a Mark IIA). Maybe the starter motor can be clocked differently, or maybe there are other starter motors that are available that would not have this clearance problem. I'm sure some Mark III owners will chime in on that.
Peter Joe