A couple of things.

How low compression?

Do a leak down test before taking the motor apart, see where the air leaks out.

Are you losing coolant from the rad?

Is this motor CIS or EFI ? If EFI, pull injector wires one at a time to isolate the weak cylinder. If CIS you will have to make a spark plug wire set that allows for shorting a cylinder at a time. You might be able to loosen a fuel injector line to drop a cylinder but it might take the pressure down for the other cylinders, have not tried it.

For a cylinder crack to drop compression much, you would be pushing lots of air out of the rad and have lots of antifreeze coming out the exhaust.