I do not want this to become a slanging match, however replacing the epa certified carburettor on a 651 with the epa certified carb from a 691 would not constitute a violation of any EPA regulations as both carbs are certified for use on the exact same short block.
Fitting a 4 barrel Hollie or even the carb from a Kohler twin then that would be a whole different matter .
In fact the twin barrel carb is actually "cleaner" than the single barrel carb as it has a smaller jet on the trailing cylinder which tends to run rich on single carbed V twins. It also accelerates quicker so reduces the amount of time the engine runs rich in order to accelerate as these engines have fixed timing so rely on a richer mixture wider throttle opening to go from high idle to full power.
As for pollution, yes every engine adds to the overall pollution load on the planet however the volume of pollution that comes from a mower engine is substantially less than from the family dog and the regulations only go to prove that the EPA is pretending to be protecting the planet by passing a lot of legislation that will achieve nothing in the long run and overall will increase overall pollution because EPA compliant products have a substantially shorter service life and the embedded pollution ( that the EPA ignores ) in a mower is substantially higher than the operational pollution the mower will generate over it's entire working life.
And I am one of the greenest people you will ever come across but the regulations on outdoor power equipment is nothing more than window dressing for virtue signallers .
FWIW
Replacing a single air flight with a train trip will reduce pollution more than never using a petrol powered mower for your entire life .
Or to put it in Texan parliance, the EPA is all hat & no cattle .