blown headgasket

i took my car to the shop and ran a diagnostic test and they said it is either a blown headgasket or a cracked block. I riped it all apart and took the head off. It looks like the headgasket could use replacing but does any1 have any other idea what could cause my car to run like shit. The headgasket is starting to come apart like it i 2 gasketson top of each other. It leaks oil in the coolant. It misfires a lot and sometimes is hard to start. Also, the car has about 85,000 miles. The comprssion is low on all of the cylinders. 1 cylinder has about 100psi, 2 of them are 120psi and the other 1 is 150psi. there is also an aftermarket muffler on it. I need some help please

compression problems will be form either worn pistons, or worn valves.

that could also mean a cracked head

im goin put new piston rings, rod bearings, headgasket and head bolts in it. The head looked good and so did the pistons. thanks for the help.