The 155hp engines never really had any issues. Although they use many of the same parts in the supercharged motors, such as hollow exhaust valves, I have never heard of them breaking or coming apart in the naturally aspirated motors. This is probably due to the fact that there is less internal stress, pressures, and heat when compared to the supercharged engines.
Seadoo had problems with the supercharged engines almost exclusively. Roughly 2007, Seadoo upgraded lots of parts including superchargers, exhaust valves, timing chains, etc. I have heard the engines in boats are a year behind. Only way to tell is by the engine numbers and/or by opening the valve cover and reading the numbers stamped on the tops of the valves.
I am not an expert on the supercharged engines though, so hopefully someone with better information will chime in. My timeline might be a little off. There is lots of information about the supercharged engines on this site if you spend some time doing a little research. Not sure about any websites that can give you a complete picture about the seadoo lineup. This website can be helpful though. If you have specific questions about years or models start a new thread and I'm sure someone will be able to answer your questions.
You can't go wrong with any of the 155hp engines. As long as previous owners did all the proper maintenance.
If you are considering a supercharged version, post specific info about the boat (model, year, engine vin, etc.) and someone can give you specific advise about that boat/engine combination.
