I don't think I can help with specific rubber parts. What was chosen years ago would be decided by the boat builder and engine installer.
I question your concern about standing the heat. Unless recently changed those pieces have done that already for , I presume, a lot of years.
If the wet exhaust system is working properly then the whole part of the wet system should not be any hotter than you can stand to comfortably hold your hand on. Certainly not more than 120oF or so.
Of course if the system is not working correctly then all bets are off but remember no part of a wet exhaust system should be hot. The fiberglass pieces, the rubber elbows and the rubber hose to the transom all have to be relatively cool.
As for the pieces then Google will be your friend or an exhaust system specialist.
Designing a Marine Exhaust System - Seaboard Marine
Marine Exhaust Systems of Alabama, Inc.
Both have picked up a good rep. from Boatdiesel. Maybe contact one of them.
ALso Trident Marine Hose is a mfgr.