[johnneilson]

The Jr. Drags and billet parts can make lots of HP.
Back some years ago we ran "stock" Briggs classes in karts on methanol.
Must be all stock parts and castings, minimum dimensions list could almost have been QC documents for production. The motors made about 7.5 HP from a 5hp model. This with stock gas tank and carb.

Back on track to the original question about porting and polishing, every motor could benefit from cleanup to elevate mis-matched ports etc.
However, from lots of time spent on flowbench with Model A motor, I would think the stock-ish motor would not gain much at all. The higher performance the motor, the more potential gain. As Brett says, the Siamese intake port causes much limitation as to the HP potential on these motors. Winfield's two up/two down crank corrected much of this short coming by having the ports firing order 360 degrees apart.

It is a good question, John