Hi I had a similar problem with my 115 Suzuki 84 model with steam & overheating problems. After replacing the thermostat,head gaskets & water pump impellor , it would still overheat.After removing the exhaust plate I found a small crack between the exhaust ports right where a bolt hole was , which tighten the exhaust plate on.This was leaking exhaust gases into the water chambers & causing water to get to boiling point a little too quick. After cleaning the ports & grinding a V where the crack was,without getting any rubbish into the cylinders, I used 2 pack putty to repair the crack & bolt hole. File finish all gasket surfaces, drill & retap the bolt hole with a plug tap . After putting this back together I havent had any more overheating issues , this was done approx 5 yrs ago & has not reoccured. I have taken the exhaust plate off twice since then to check the repair, but mostly to clean the water passages in the exhaust plate , which can block very easily on this engine . I do flush my engine after every run but I do believe preventative maintenance may cost me less in the long run. I hope this was useful to you.