I mowed for 90 minutes and the engine just quite on me. I thought I might have hit something, but everything was clear. When I went to start it back up it wouldn't fire. The electrical is fine, turn the key and the starter spins the engine, but no combustion seems to be happening. Could it be the spark plug?

Could it be the spark plug?

Could be but not likely. Spark comes from magneto to plug, not connected to other electric. Most likely something grounding mag. Trace kill wire from mag to first safety switch (most likely parking brake.)

Hi,
Most likely a bad armature, module, coil; depending on manufacturer for their preferred name.
Peace,
Clarke