Misfire at cruise, popping/afterfire at 3000RPM

Yes; springs can take a set. But those would have to be highly stressed.
The valves that were open for 20 years could have rusty seats and and so, they do not seal. Also the Guides could be grabbing the stems.
These things often happen to engines not bagged. The moisture content in the air all around you is constantly changing with temperature and barometric pressure. Your Crankcase is constantly breathing.