1994 Chevy Astro slight knocking noise and vibration from e

I have been having 'bucking' problems with my astro. So far I have had the top hose changed (cos it was leaking), the rotor arm, distributor cap, leads, plugs, ignition coil and vacuum hoses changed. I just had a mechanic out to have a look at it, because although the bucking has calmed down, the engine sounds like it's running rough and I have a sporadic shaking of the vehicle. He states that the problem is a cylinder misfire caused by a damaged piston and that this would involve an engine out job and either a full stripdown or a new engine. Surely with only having 67,000 miles on the clock, this shouldn't happen, as I know that these engines are built to last. Any theories on what else may be the problem before I shell out thousands on fixing it? Thanks guys.