Kohler 27hp

My dad has a DGT 6000, Craftsmen lawn mower. He knows nothing about the history of this mower. The PTO switch was missing when he bought it. He replaced is with the correct switch. Now when he starts it when you let the clutch/ brake pedal up, it dies. It dies as well with the PTO switch is pulled. He has tested both safety switches, the seat safety switch and the Parking brake switch to start the engine. What could possible be the issue? Any help on this matter would be greatly appreciated.

Most likely related to the seat switch. Check any relays that could be failing causing it to think nobody is sitting in the seat.

The problem is either the wrong switch has been fitted or the wiring is all messed up.
No option but to get a wiring diagram and go through the mowers wiring.
PIA , tedious job .
Use the 917.?????? number on the tag under the seat to download the owners manual from Sears.
In it is the clearest most easy to understand wiring diagram you will get for any mower.
The PTO & Seat switches work together as a circuit
the brake/clutch switch & the seat switch work as a circuit
Both of the above work in parallel.
Both of them are shutting down the engine so the prime suspect is the seat switch or the wiring to it.
Nothing wrong with the engine problem is in the mower.