Maybe you can just replace the brushes and the regulator. On my 240 I was able to pull the alternator and remove 2 screws and remove the regulator/brush holder. In most cases it is just a matter of worn brushes. The easiest thing is to replace this internal part (as long as the bearings spin freely and quietly). It takes all of 5 minutes to change out these parts. Take a little crocus cloth, polish the armature, screw in the new regulator, reinstall, and back on the road for less than 100 bucks. I've never done the repair in the 850 but the alternator should have the same configuration as the 240...I'm sure if I'm wrong someone will post for sure !!!
Good luck,
Jack W.
PS...another alternative is a boneyard crash victim.
If the part looks relatively new and clean you might just luck out and save some cash that way !!!