This mower starts and runs fine, but when I engage the blades, it stalls. The seat safety switch has been disabled. The belt is on correctly. Any help would be greatly appreciated.

Do you have a "reverse" switch on the unit? Those that do have to disengauge the blades before backing up. If you have one it may be stuck on or there may be debris holding it in the "on" position.

You unit likely has 3 safety switches and please tell us why you are disabling any of them?