Get a compression test done on it. If it's even and over 100psi across all of the cylinders then it's not shagged. If you have one down then there's a serious issue.
You should test it when cold and when hot. If you're a pot down when hot then it's likely there's a crack in the head.
If you're not down on one under any conditions, re-test with a squirt of oil down each bore. If the pressure rises significantly when oil has been admitted then you're getting blow-by past the piston rings, so the bottom end is failing.
If none of these varies stable results (and they're good and over 100 PSI) then look to your fuel system and check the system pressure and health.
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