If you do a decarb with seafoam it will clean the reeds? If the engine is running get a can of spray seafoam and bring the engine up to temp first then
Two things you can do if it's "mechanically sound" (so not the prop etc).
1) install a thermostat (with a gasket - not included)
If that still doesn't help you, you can look for a blockage in the water channel to the heads.
The tell-tale gets it's "feed" from
Good question did you change oil regularly and did it have good oil pressure before you took it apart?
Valve stem seizes to the guide from sitting too long? Then piston hits valve?? Is this the theory?