I have a Simpliciy Regent riding lawn mower and when I release the clutch/brake, it stalls. I tried to hotwire the seat safety switch, but that didn't work. Any suggestions?

Hi,
I am not quite sure what you mean by hotwire, but the seat switch would normally be jumpered to check. The starting point is to disconnect the ignition ground from the machine frame to the engine. If the problem goes away, then it is in the wiring of the machine itself (safety switch or bad connection) and should be explored there. If it does not go away, it is probably the drive system (belt, pulley, etc.) at fault and should be explored there.
Always best to post model numbers if you can find them since I can fine manuals for most, and Simplicity has a voluminous site of manuals.
Peace,
Clarke