it's all good. if the motors are sound all else can be worked out. it cant have stats in it as there would be no water flow to the exhaust until they open. at this point you really need to figure out everything you do have and what you dont. it may be missing a sensor etc or the ecm may have or may not have been reworked. you are going to have to establish a base of what should be on it and what you are working with. personally I would go back to circulating pumps and thermostat housings and work from there but it may be better to pull the intakes and go to carbs if the ecm setup is too messed up.

You know it hadn’t even crossed my mind to ditch the fuel injection and go to back to carburetion. May be a lot more pros there than cons. Previous owner wanted fue injection for reliability,(his words not mine) but I don’t believe he was the one doing the wrenching on it or even capable of doing it.

Over the years, many of the 502mpi systems have been converted to carb.
Not everyone agrees going to carb is backwards. I even did it. No remote start anyway, so that’s that.

the plugged vacuum line goes to what appears to be a fuel regulator. Another line goes to the intake. Then it “looks” like the fuel lines run through a “log” that the water pumps water through? Is that correct and if so is that to warm the fuel?

sorry I built my first motor at 16, actively drag race etc but don’t know squat about marine MPI but love to learn

TIA!

Make sure there is no ect modifyers between your temp sensor and harness, procharger used to supply those to trick ecm into getting extar fuel before anyone could tune mefis. Twin engine, they both do exactly same thing, mefi1 ecm or mefi 3?

Ahhhhh, picture is worth a thousand words.
Your engine had a procharger mounted.
Part of their package was a water crossover that replaced the factory circulation pump. Chances are no tstat , or one that was been drilled out because water needs to always flow to exhaust.
Your fuel system may have wrong psi’s for naturally aspirated fuel psi (ati’s FMU is still installed) and always possible someone has done ecu tuning.

Will do. How do I know what ECM I have? Like what generation or year this set up is? Where is the model and serial number of the engine should be on the computer box, it’s blank.

Few pictures and GM MEFI numbers in link.
https://obd2allinone.com/mefitune.asp

no tstats! I’ll start there thanks