You can load the spark plug hole with priming gas using the soda straw and finger method but alas, with the comp reading your seeing it will never pop.
If it ran before I would not be looking towards the decomp valve.
It would be nice if if you had another head to swap on.
Put the head gasket back on. Unless you lapped matched the head and top of the cylnder, that going to be a positive leak compression leak.