Bill,
At 20 years and less than 400 hours, I can make two suggestions.
The first and simplest is that the rings are gummed up. A couple of lube oil changes MAY fix that in about 50 running hours.
The rest are not simple given the service resistant install.
It may be that the valve guide seals have dried out or died. That is about the age that rubber parts start going bad all by themselves. The oil change may help this.
Last and not a suggestion is that there is something actually wrong, like a cracked piston.
I have seen these installed and they are not fun to work on.
Matt