Okay, the first step is for you to remove the two pulleys that are welded together (the stacked drive pulley) It's just stuck. it will come off, you may have to resort in cutting it parallel to the PTO and splitting it open to get it loose, but you may find that some penetrating oil and some time will allow it to slide off easier.
After removing the pulley, do us all a favor and continue to disassemble the mower, install better steering, better brakes, bolt the wheels on (snap rings alone won't work) aand install a tethered kil switch after that has been accomplished THEN and only THEN change the gear ratio to make it go faster. And then before you drive down the street... get your safety gear on, helmet, long sleeves, pants, boots, neck collar
I have a feeling that the moderators will be locking this thread down soon. We really do stress the importance of SAFETY and doing things like a simple pulley change and tearing off down the street to see how fast it goes is something that is frowned upon severely here. It will get you banned from the site. So if you want to contiue to get help, you need to make sure we understand that you are just as rule oriented and safety conscious as the rest of us.