I had the exact same issue a few months ago, right before a two week holiday on my boat. The pipe coming off the back of the exhaust riser coroded and gave way with straight exhaust coming directly into the engine bay - not pretty!
One of the three bolts was seized and access (as you know) is terrible. The design of the engine compartment is certainly not mechanic friendly.
I ended up cutting an access hole at the back and would reccomend doing that. I then replaced the pipe with galvanized pipe from a plumbing supply place. It seems to have worked out fine so far.
I also cut an access hatch through the galley counter top below the companion way which lifts out of the way over top of the engine. Just need to slide out the drawer and it gives me pretty decent access (relative!).
Hope this helps.
Will
Olson 911S 'Targa'