Ok... It is an injected scoot so it has to have a return line from the injector. That alone should be enough to prevent Vapor lok in the fuel system but IF the return fails, the fuel system will continue to build pressure because the vent is designed to allow air into the tank, and seals it from going out. When you open the cap that back pressure is forced back into the tank thru the Primary fuel line creating the Gurggle you here. I believe the failed return is forcing a super rich charge that has overheated the cylinder, and created a head gasket leak for the coolant to escape. Basically a blown head gasket due to a failed injector return line. Hows that sound?
The first easy test is to have a garage pressurize the cooling system to verify the leaks location, and next to run the injector system to verify flow into , and out of the injector port.
I'm pretty sure about this one. Injection is much easier to troubleshoot than a carb system, and I am certified in marine fuel injection both gas/diesel
To top it off the whole issue COULD have been created by "bad gas". IE: Phase separation/water. Injector nozzels are almost micro sized so one tenth of a drop is enough to TRASH an engine. Sorry if I'm right!