I have put some time into thinking about whether to do just what you have suggested, but I figured that there are definitely more 750's out there than 850's (well there seem to be from what I have seen), and seeing as though I have the complete gear train, casings etc for another 850 - I am hoping it should be easy enough to assemble it using the parts from a 750.
I picked up the 750 today, and she looks to be all there. Will start stripping her down to see what the internals are like, and their suitability for the 850 program.
Here are the pics of the 750 as she stands today. She is an Australian labelled Pioneer, and as can be seen - has had a chip flap riveted to the clutch cover during its previous life. The muffler has seen better days, but will still work okay. The current owners father in law found the saw in a rubbish bin, and decided to save it from a certain death. Lucky for me!
That chip flap is OEM. The 700 clutch cover isn't setup for it, but the 750s has rivet holes.
All the parts you need will interchange. The differences between the 750 and 850 are the PTO half crankcase, crankshaft, gearcase, and wrap handle. The flywheel half crankcase, top end, rear handle, flywheel and ignition are all the same.
Chris B.