The cyl. head on any internal combustion engine is certainly "under pressure"
If you have a leak in a water jacket, you may be able to repair it with JBWeld on a temporary basis, but I wouldn't do it.
The head will need to be replaced.
You can try WD40, or PBBlaster as a penetrant to loosen the rusted nuts. Apply liberally and often over a period of time (days). Use an electric impact and proper socket... you may have to heat the nuts gently with a torch(don't over do it).
We are not primarily on earth to see through one another, but to see one another through
Some people are like slinkies: not really good for anything... but you can't help laughing when you push them down the stairs