Thos motors are strong as a brick poohouse in my opinion, I did have one chuck a rod once and that was thanks to an owner that never checked the oil. I replaced it with another engine of the same type that was even older (mine was white and circa 1974) and it has outlasted the rest of the mower (the mower I pulled the engine from was worn out as well)
As to the carb, I would pull everything apart from it gently and leave it soaking overnight in either fuel or some heavy duty degreaser, try and clean every orifice you can find in it, blow air from an air compressor through every gallery and refit with a new diaphragm and tank gasket (around 10 bucks) then check the tune and go from there.
