I don't think replacing the belt is going to solve this issue. Take the mower out to where you have a little bit of room. You may have to lift one back wheel off the ground to get it to roll. When you have it on a relatively level surface with plenty of room make sure the bypass lever is pushed in. Then crank the engine and run it up to full throttle. Place the shifter in neutral (with foot off the brake) and move the handle slowly forward as far as it will go. When it's all the way, count to 4 then slowly bring the handle back to reverse as far as it will go and count to 4; cycle like this several times. If after about 10-20 cycles is has not started to move or move better we may have something major wrong.
If that does not work for you the brake may be stuck. Now that I think about it, check the brake before you do the previous procedure. If you look on the right side of the transmission (you will need to get down on your knees low to see it) you will notice a rod going through a flat lever piece and a disk sitting to the left of that. Those are all parts of the brakes. Make sure the parking brake is not set and use a screwdriver to push the flat lever part forward. If you have someone that can help you with this it will work better to already have the tractor starter and foot off the brake/clutch then push the lever forward. If the tractor drives fine then, it's just a brake sticking and the lever will need to be oiled. If oiling it does not fix it, you will need to lightly sand the rod where it goes through the lever then put some oil on it.
Let me know what you find and we can proceed further if necessary.
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