Jeez, like I said I’m new to stand ups. Also in the middle of a work day and last thing I care to do is try and decode some smarta$$ comment lol. Was really hoping for some better advice from this forum...
It was funny and you know it. Can't always expect serious answers. The forums are just the beach without the sand. Some good advice, a lot of bad advice, and a few good (and bad) jokes.
You can buy an infrared thermometer at harbor freight for $5. Check the temps on your head, cylinder and pipe. It might not be reliable on polished surfaces. You can also try flicking water on the parts and see how it reacts. Head and cylinder should bead up, *maybe* on occasion create some water vapor but for the most part, no. Water on aftermarket pipes *that are not jacketed* should create vapor, sizzle on race boats, never boil. Stock and jacketed pipes should never sizzle, and very rarely make vapor.
Check for blockages by running it on the hose. To do this you need a flush connection. These connections however are in the middle of the cooling and the pressure will go the path of least resistance. You can block the pisser but you can not block the exhaust injection or the intake side. You can cut a pipe of plastic tubing (I used an old Kawi fuel pickup tube, fits perfect in aftermarket pissers) that you can shove into you pisser (team ram rod) and "backflush" the ski. That reduces the outlets from 3 (out, in and exhaust) to 2 (exhaust and in). Observe the flow of each area and decide if adequate water is flowing. Always flush with the ski running, otherwise the exhaust can fill and backup into the motor. Back flushing is actually better, doesn't require an installed flush kit, but is much less convenient.
This is a start since you asked specifically about cooling.
There are a lot of other issues which can cause such symptoms. Bad tank vent (as fuel is drawn, it creates a vacuum, without an inlet source, the tank pulls vacuum and starves the carbs). Failing CDI/stator/etc (caused by heating, but they don't have cooling systems).
No information about the starter conversion here, but SBT starters are only good for about 1-2 years. Contact the seller, see if it was an SBT starter, because I think they come with a 1-year warranty, but I may be mistaken, or that may be optional extra $$.