I don't know how we got onto a crankcase is the same, from talking about mufflers.
However, I think that a 91FZ crankcase will work on the 91AC. I'm not 100% positive but you can can YS Parts and service and ask them. There may be some other misc parts you might need to convert over to the 91FZ case but I think it might be possible. Richard can tell you the specifics.
you can get their contact info from
www.yspartsandservice.com
Troy Newman
Team YS