You're right, there's a lot of work in replacing a bulkhead (half or full) in a sled but it's doable and the cost of parts isn't outrageous and none of the powertrain is involved. I haven't done it, but I'm familiar with non-AC Yamahas.Only speaking about the arm attachment, as it looks all too familiar.
Most modern snowmobiles are very similar in bulkhead design. There are reinforcement kits for all of them, and are needed in that application in my opinion. Broken bulkheads are VERY common in the sled world!
Simple enough to fix, but it’s labor intensive!
If you're adding in the diff housing and r&r'ing all the parts in it like this appears to be -- that's a different ballgame.