Maybe the breather needs to be replaced ??

You think? LOL

I feel a bit silly but it is only fair to own-up. I checked the oil level again and to my surprise the crank case was filled with gas I could not believe my eyes. When I was checking the oil level it looked like there was a bit of oil due to the gas only clumbing at the end of the dip stick and the rest of the dip stick was nice and clean. 1st time I have seen this. The good news is that the engine is spotless inside. I guess there are more surprises to come. This why I love servicing stuff when you think you have seen it all YOU Havent. Later

Nothing like a little Napalm on your Dipstick ta make yeah feel alive Roflmbo !

Yas, and you won't believe yer ears when the engine goes "BANG" from all of the fuel that has thinned out the oil, and filled the crank-case!
Better get a fuel shut-off valve and use it every time you turn off the engine for more than a few minutes! There is a certain discombobulation that affects yer mind when it goes bang, especially on a rear-engined riding mower! BTDT!
Doesn't the oil smell like gasoline?
And, check for a bad float in the carb, or a dirty seal where the needle valve goes.

Rats. I was wrong about the vent. Oh, well, win some....
Anyway sometimes beating on stuff works - drop the carb side from a 6" height and see if it un-sticks the float. Change the oil and maybe the air filter if it's oily. And just for insurance, put in that shut-off valve. The Tecumseh has an 'elevator' arrangement that throws oil to the upper bearing. Since the fluid in the crankcase was quite high and thin, this probably accounted for the big leak. At least the thing telegraphed that it had a big problem - it pays to pay attention and not ignore symptoms. This symptom gets reported once in awhile so I'll keep it in mind.
A possible preventive for all kinds of carb trouble is the use of 1 oz Marvel Mystery Oil/1 gal gas.