Is it vacuum bagged? Some time ago Lycoming started cocooning the engines in a foam covered thick plastic bag. If the engine was stored in a dry climate, never rotated and still sealed, chances are it's OK. I don't know if the owner would be too keen on you pulling a jug on a new engine before you buy it. That engine 20 years ago from Van's was probably about 18K, given that, if the engine can be had for that or less, than I think you can't get hurt.