It's probably a good idea to remove the exhaust elbow
as well anyway. It makes it easier to get the core out by pushing it out instead of trying to pull it out. You can then inspect the elbow which should be done anyway and the gasket
kit Yanmar sells for the heat exchanger comes with gaskets for both ends. Trying to put the core back in without both ends of the heat exchanger removed becomes a game
of chance. The probability of the Y shaped gasket
falling off inside the heat exchanger while pushing the core from one end only is highly probable. As well, the end cap has to be removed to properly replace a new O-ring which should be done as long as you're that far into the job.