I would also assume the battery and starter are fine. Jumping across the big solenoid terminals will confirm that, less the possibility that a terminal connection may be intermittent.If you haven't cleaned the battery and solenoid terminals well plus check that they are secure, now is the time. One side of the solenoid's small terminals is activated w/ 12V when the key is turned, the other is grounded when all the safety switches are in run mode. So, yes it could be an ignition switch, but you have a few things to eliminate first. Your owners manual may have a schematic or they are available on line.

Think Timing... For power to be developed, the ignition of the compressed air/fuel mixture must take place at exactly the correct instant - just before the piston reaches Top Dead Center (TDC) on the compression stroke. Timing is set on older mowers with point type ignition systems by adjusting the point gap and generally only changes due to wear. However, these changes are gradual and unless the points come loose for some reason, will not likely suddenly prevent the mower from starting. On newer electronic ignition systems, there is basically no adjustment as the position of the electronic ignition coil/module fully determines ignition timing and this is fixed. However, timing can be grossly messed up if the flywheel key gets sheared and the flywheel then rotates a fraction of a turn on its mount on the crankshaft. The result may be a mower that does not start, backfires or runs erratically, lacks power, won't run and/or start when hot, etc. This is very likely to happen should the blade strike a rigid object causing the mower to stop instantly. In this case one or both of the blade lock key and flywheel key have sheared to (hopefully) protect the very expensive internal parts from damage. There are likely not going to be any timing marks for that old timing light you have sitting gathering dust somewhere. The only test really is to inspect the flywheel keyway to determine if damage has occurred.

***" But the problem is in reinstalling the 4 brush plate and holding all the brushes in to get past the rotor."*** That part (referred to as "rotor"), is the commutator (in case you do try taking the starter apart). There are a few threads throughout this forum that include some methods for "keeping the brushes caged" during assembly. If you are going to attempt a "teardown", I suggest you research the older forum threads for these tips and techniques. ***" You can field test the starter on a bench with 12 volts."*** Yes, but not "just any" 12V source. Ideally you need to use a 12 volt automotive battery, at least as "big" as what is in the tractor, and that battery must be fully charged. Trying to use a small, low output battery charger will likely produce negative results. Use a set of jumper cables to a "known to be good" 12V battery for bench (or "driveway") testing.

Put a glove on your right hand and when the tractor is not running reach under the right rear of the tractor(from back) and push on brake actuator(flat piece of metal mounted north and suth of tractor higher in front then back)... this will release your stuck brake....then go to garage with tractor and get 1/2 inch wrench and loosen brake adj nut in same area. only maybe 1/2 to 1 full turn...recheck brakes to ensure they still have enough "grab"