I'm not sure on the year, but the 13976A was a nonlighted 4hp model that used Tecumseh HS40 model 55279B and the tank would have been mounted on the engine.
It would have also used 4.10x3.50x6 tires too
This is interesting and something I've never noticed before. Apparently, the MW models were color coded by the model number.
It says the 3hp model was Red, the 4hp nonlighted and lighted were Sunshine Yellow, the 5hp single speed was Brittany Blue and the two speed 5hp was black.
Hmmm...