Matt
If doing the work yourself I would remove the 850 leave the oil in it and wrap it up and put it aside for safe keeping.
Take an 1100 motor (mini K) 4 speed syncro box and paint it like your 850 motor. The mini K ran a stock 1 1/4 carbie and air filter but was visualy different because it ran an altenator. you can still run the generator with the 1100 if you like and most people will never know and it will look exactly the same as your 850.
Just remove the heater and there are basically the same.
In saying that the 1100 would run so much better with an 1 1/2 carbie & extractors (all minis need to breath better) and run a set up silimar to Bruizer with the mini matic larger carbie and metal air box. I used this as I didn't want to run twins and wanted to retain the standard mini look (matic) as I didn't want a sports filter.
Bruizer also has the extra steady as it retains its original wand gear lever. If I didn't add the extra stabilizer the motor would go through engine mounts at a scary pace.
Larger head, bigger valves extractors and 1/1/2 matic set up nice
Any questions just PM.
steve