One prerequisite to a good compression check is a valve adjustment. If the engine sat for quite a while without being run, sticky vavles are a definite possibility. If you can clean up the damaged spark plug threads without pulling the head, reassemble it, adjust the valves, and make sure it's getting enough gas, then drive it a while. A little crankcase additive like Marvel Mystery oil, or some other lightweight, solvent-type additive will clean up sticky valve guides with a little run time. It's possible the blowby will also diminish as the piston rings get run in to better contact with the cylinders again. If not, a little more run time won't do enough damage to affect the success of a rebuild.
Jerry

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