My 1954 Johnson Seahorse CD-10 has been in my family since 1964.  It hasn’t run in about 50 years.  It appears to be in good shape from the outside but a compression check of both cylinders showed that it has 35 psi on both cylinders.  Since the power head isn’t seized, I thought that the rings might be seized onto the pistons.  I poured Marvel Mystery Oil in both cylinders (with the pistons pointing upward) on three different occasions and let it sit for awhile.  The last time I waited for two weeks…thinking that the rings, if they were seized, would free up.  No change occurred with the last compression test.  I decided to pull the head and the exhaust chamber cover and look at the cylinder and piston surfaces (at least see what I could see without tearing the power head apart).  The head bolts were not difficult to remove and the head came off with just a slight tap of a mallet.  The head gasket looked okay with no obvious signs of gasket damage.  The exhaust chamber cover came off equally as easy and its gasket looked okay.  I did not see any significant scoring of either cylinder or piston surfaces.

Is it possible to have poor compression (with both cylinders having the same compression readings) with just a head gasket leak?  Will a leaking exhaust chamber cover gasket affect compression?  Any thoughts or recommendations?