First, IF YOU VALUE YOUR LIFE, DO NOT just pulley swap it. A stock riding mower is built to go 5 mph, so that is why everyone on this forum have all changed a bunch of parts out to kart stuff. You cannot switch your current pulleys, and nor should you if you plan on leaving it stock. It will do around high 30s to low 40s with those sizes. Make sure that your transmission is a manual, and remove reverse (lever going to the trans case that you can see moving as you shift the mower, and reverse is because it will break and break the case). You would be safe to do that if you did some basic things for safety. For example: go kart front end, better brakes (a pit bike brake is usually the cheapest option for decent braking), tether kill switch, actually putting gear oil and a locker in the trans, and maybe smaller diameter rear tires for better stability.