76 mercruiser 4 cyl in/out water in oil

by Joe
(N.B. Or. U.S.A.)

Hi I am Joe I bought a boat with problems.Its a 4 cyl 120 mercruiser.The oil filter was full of water.It had a stuck exhaust valve in#3 cyl.It has 120 compresion in #1 #2.and 90 in 3&4.The engine has not been started in 6 yrs.Also #3&4 hit 90lb. in 3 cyl. cranks with pb blaster.
how can I find if head needs to come of?