The wikipedia page on the 914 2.0 states:
70 kW (95 PS; 94 hp) (US)
74 kW (101 PS; 99 hp) (RoW)

Thats a significant difference. Is that due to the RON 91 petrol used in the US, or are the smog regulations the main reason. And if it's smog regulations, is there a way to 'undo' them?

Go to page 97 of the Porsche Panorama (Porsche Club of America magazine ) January 2020 #754 there is an story about the difference in HP ratings for USA and DIN. The Society of Automotive Engineers uses one calculation SAE and the Deutsche Industrie Normal DIN uses another . They used two different methods but not true anymore .

Euro pistons add about .5 of a compression point.

all you have to do it tear down the motor and change pistons for 5 hp!!!!!

Yes, US pistons have a 7cc dish for a compression ratio of 7.6 to 1 vs the ROW had flat top pistons for a compression ratio of 8 to 1 and 100hp. That was the magic number Porsche wanted as the 914 2.0L really was replacing the 912 and the 914/6.