Depends on why there is no compression.
When it was new it had 150 PSI. per cylinder
That engine FRESH / NEW should have 150 LBS. compression per cylinder. If you have more then 20 LBS. difference in any cylinder then it is not in good shape and a good VALVE JOB is probley in the furture.
Just about to fire no 1 w/engine at TDC on compression stroke
A new engine would have 150 LBS or more per cylinder. A used engine that runs real good and that has plenty of power should have at least 140 LBS. per cylinder. If there is more then a 15 LB. difference in any cylinder then you will have a rough idle and maybe an engine miss at a idle. That means there is an internal engine problem.
maybe you need a new head gasket?
No, quite the opposite- the idea of gaskets is to ensure that an engine gives HIGH compression. If your car has just had a cylinder head gasket change and you are STILL getting low compression, then this suggests that the problem lies with the cylinder head itself- it may be cracked. Unfortunately there's no effective repair for this, the only solution is to get a new cylinder head I'm afraid.
Check the Piston Ring.
bad valve, bad rings or head gasket. need to do a leakdown test and see where the cylinder is leaking. do a copression test with adding a tablespoon of oil in the plug hole. if you get some compression, it's rings. have same problem with 5.0 in f150. mine bad rings on one cylinder. probable cause would have started with a bad injector leaking fuel and washing oil from the cylinder. then on restart that bore would not be lubed causing premature ring wear. at 150k, time for a new engine.
Unless it was built with high compression and or a "big" cam causing high cylinder pressures a stocker should suffice .
It would largely depend on why #6 has no compression. It could be a minor repair or it could be a major repair. If it is a major repair and the engine has high mileage it would be wise to replace the engine only if the vehicle is worth saving.
In an internal combustion engine, there are cylinders that have pistons in them. When the piston is down you have an open cylinder that has a specific size of volume that it will hold. When the piston moves back up to as high as it can, it compresses all that space into the new space and that is the compression ratio.