That is just about the worst result possible. You have enough expense in front of you, before you start to re-engineer Amel re. the exhaust etc. could you ask your technician to asses just how much of that damage would be as a result of all the cranking that has gone on trying to start the engine.
If the the engine wasn't starting the raw water had to keep building up in the exhaust and backing up to the engine. There have been thousands of marine engines - especially in power boats destroyed that way.
At least you now have a fair idea of what you are dealing with regarding the engine, I would respectfully suggest to review the suggestions re. the exhaust system.