pair of new plugs
pair of in line spark testers
Can of carb cleaner.
Put the new plugs in and install the spark tester where you can see them easily.
Go mow, the instant it stops spray a VERY SMALL shot of carb cleaner down the carb and crank the engine while watching the spark testers.
IF the testers are flashing the electronics are good
If the engine fires on the carb cleaner then the valves are good which only leaves the fuel supply.
IF it fired the do the same again but before the engine stops give the carb another shop.
If you can keep the engine running like this for 30 sec or so, time to check the entire fuel system, from the carb through to the fuel tank & fuel tank vent.
Pull the fuel line off at the inlet side of the pump loosen the fuel cap and blow back through the fuel line a couple of lung fulls.
Reconnect & try to start the mower, runs fine + obstruction in in the fuel line between the pump & the tank.
ZTR engine eat dust,
For fuel to get out of the tank the dust laden air has to get in.
Add some grass clippings and you have a plug that can block off the fuel supply just enough to be unable to keep up with the engine.