There aren't that many places, right?
I would check the entire case circumference, every mating surface. Both crankseals
Intake gasket area
Exhaust gasket area
Base gasket area
Head
Even if you plug the cooling passages, you would still have a leak (if that's where it is), because they will be at ambient pressure, and your cylinder will be higher than that.
If you really suspect that's where it's at, take the head off and inspect the O-rings. When you put them back on, put some vaseline on them, too.
I am sure one of the site pros will chime in - surely they have experience with gremlins like this one.