If he can get the parts, I don't think he'll have any issues fixing your 850. He had no problem turning my 861 into an 861SE for me. The SE upgrade is just a transport, so its really the same exact job.
In the mean time, if you come across a good used 302, its definitely a nice upgrade from the 850 (at least I think so). Also, if you would consider just a dac, Wadia's new products are outstanding, and the prices are very fair for what they give you. They brought some of the original Wadia people back and they're off to a good start.