Hydronic heat is a great system when it works, but I’m not convinced its the answer. Ive spent way too much time, effort and $$$ to keep our Hurricane running. By far, it is the most time consuming system we have on the boat. As old as your current system is, you can pretty much count on the whole system needing to be replaced. We just had all the hose replaced, over 250 ft, because the old hose was dry rotted as well as new circulation pumps. So before you jump on a less expensive heater, you might want to get quote’s on the entire system, not just the heater.
Best of luck on what ever you decide.