63 VW Bug

I'm having a problem in the rocker arm at the #1 cylinder.
The pushrod disengaged from the rocker while I was driving.
Both rockers at this cylinder have greatly increased clearances
as a result. ( say maybe 1/4 inch) Too much to compensate
for with valve adjustment.
The valve spring does not look broken.
When I rotate the engine by hand all seems well.
WHAT CAUSED THIS ? HOW DO I FIX IT ?

Which engine is in it?