So sorry to hear about the engine failing. You could re-power the tractor with a replacement engine. The 3T80J, 3T80, 3T80U, 3T90 all would bolt up to you JD850.
These Yanmar/Deere machines are so neat, one can literally split two of them swap the front or rear with the other and be back in operation again. No need to even pull the engine.
So, with parting out the engine is just fine. Once you find another 'like' Yanmar running engine in a machine to mate the rear with, you are back in business. See this link on how simple this task really is.
New guy here from Georgia. Just thought I would share some pictures of my recent engine swap. Front end: YM2000 Rear end: YM2210 The 2210 was newly refurbished but, through a series of unfortunate events, had serious engine issues (long.... stupid stupid story). She was non-operational when I...
www.tractorbynet.com
And, I'll pass on your info at the YTOG.