Changing Pulleys On my Craftsman 12 hp Brigs and Straton?

YOU can for sure do this, you can put any pulley size you want and pulley with these measurements are readily available. Same with the transmission end, as long as you buy a pulley with the shaft and key-way sizes that match then you are off to the races. i really can't tell you what sizes to buy specifically, but as you do what you are doing remember the bigger the pulley on the engine, the more horsepower it will take to get you off and going. If you go to much bigger you will need to figure on how to get some more horses under the hood.