when i bought my Nordie i also had a slightly leaking head gasket.
But in my case the water moved to the outside of the cylinder.
So i added some Nm's to the 4 head nuts
In the end i used 40Nm if i remember right
No leaking since then.
Maybe its worth a try
But be carefull.
Years ago a Super Mono racer from Denmark has tested the original studs.
But i cannot find his numbers.
If i remember right he told that adding about 10% will cause no problems
Again be craefull. There's no waranty
If i understand right your problem is leaking gas into the water cooling circuit
Here around there have been 3 cases where Nordie riders had the same problem.
Maybe it's caused by a weak headgasket. Usually they had no problems, but squeezing the right grip very hard the motor blows gas into the cooling system. They lost liquid out of the expansion tank.
The idea was to replace the original head gasket by something from solid metal
I suppose Pete has some experience doing this
ciao Joachim