Its been years since i needed to rebuild an engine, but its either going to be a valve or head issue maybe camshaft problem, or a piston issue, yes it could be be a cracked bore and i have no idea if the liners are replaceable.
At one time any decent motor engineer would have been able to whip the relevant cylinder head off and diagnose the problem aand give you a verdict and estimate to fix, entirely possible a cylinder head rebuild or replacement cyl head would fix the issue.
Basically find yourself a good engine specialist, or decent commercial mechanic and put the job in his hands you have nothing to lose other than to pay for his time to diagnose the issue, even if you then decide to sell if you can say exactly what's wrong you have more chance of selling on to someone who can fix it.