I would agree. At this point trying to rebuild this engine, if it indeed has failed as described, would not be the way to go. Your engine failed so my opinion really doesn’t help your issue, but I would have to strongly disagree with your mechanic’s assessment of the Kohler Courage Twin cylinder engine.
The problem for the Courage Twin Cylinder engine is it shared its name with the Courage Single cylinder engine which was without a doubt not Kohler’s best work. But’s it also why Kohler took care of failed Courage Single Cylinder engines far outside of their warranty period. Many years after in fact.
Like I said, yours failed so this really won’t mean much to you, but the Courage Twin Cylinder engine was a good engine. It wasn’t without a few nuisance issues when it first came out, but it was not known for major failures.
I’ll admit I am somewhat biased as I represented Kohler for 27 years, but I also have nothing to gain from defending them. Like I mentioned before, the Courage Single Cylinder engine gave Courage a bad name and too many simply hear the word Courage and regardless of whether it’s a single or a twin cylinder it gets tagged as junk and nothing could be further from the truth for the twin.
Whether it's a Kohler, Briggs, Kawasaki or Honda, if an engine runs from brand new for an hour without issue, and then has a catastrophic failure at 300 hours it is rarely the fault of the engine.
I will step down from my soapbox now.