One approach is to take a (non-leaking) gas-fired hot water heater and
pipe the exhaust to go up the stack of it (in place of the burner gas
flow). You could do the same with an old oil-fired boiler, or with a gas
instant hot water heater exchanger.

You can probably find something in the boat industry, but it might be
just as fast/cheap to get one tigged up. Best if it breaks the exhaust
stream up into multiple streams, and the closer it is to the headers,
the better. Before/instead of the muffler, not after it, preferably.

One of those stainless steel flat plate heat exchangers would probably
work fine (if a large enough one was selected) until it coked up - they
are not great for cleaning.

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