The engine starts fine, but soon as I try to put it in gear or engage the blades, it stalls out. I checked the belts, all good, replaced within the last year. Blades clear of debris. Not sure what to do next.
Sounds like a short on the seat switch or a duff seat switch or a short on the seat switch wire.
Safety switches that cut out the engine are a series of individual circuits that are all wired in parallel
They are
seat + brake/ clutch
Seat + blades
Blades + reverse (optional)
As your mower stops when you dissengage the brakes or turn the blades on, the common item to both is the seat switch.
The switch rarely plays up because they are up away from the dirt dust & weather but the wiring does and it regularly gets pinched.
All these safety switches are ground circuits and they ground out the magneto so you test them by continuity to ground with an Ohm meter.
Do not use a test lamp connected to the + side of the battery as if 12V even looks like it will connect with the magneto kill circuit, you will kill the magneto , stone dead $$$$$.
Start by testing each switch without the plug they should open & close.
Most are double switches with 2 pairs of contacts the switch works between the contatcs that are flat to flat not edge to edge.
One side carries 12 V for the starter solenoid ( usually different coloured wires each side ) and the other is the safety kill wires ( usually the same colour each side )
Then unplug each switch and check the wire from the contact to ground and contact to contact.
Ground shorts are a pia to find.
