spark plugg are burnt

martin53,
Can you better explain these "adapters". I think you are confusing people. I am guessing the spark plugs were screwed into "non-foulers" which stand the plug back so it doesn't get oiled. Those are usually for engines with bad rings that are on their last leg.

I don't think leaking valve stems will foul plugs, unless severe. Usually, it allows oil to suck into the intake while sitting at a stop light, then you get a puff of blue smoke as you drive away.

Don't go by what a prior owner did. Evaluate the engine yourself before deciding on a fix. The engine could be fine and their thinking was screwed up.