The part that generates spark is call a magneto by most of the industry. Briggs calls it an armature.
Shorting seat switches out to test for spark is not always a valid test of things due to how most of those switches work/are wired.
To determine whether there may be a short in the kill wire circuit (wire chafed or mouse chewed insulation) you should remove the small black wire directly from where it connects to the magneto itself and then recheck for spark.
If......with that wire unplugged from the magneto....there is still no spark, the magneto is defective.

Another thing to check is to see if there is rust on the legs of the magneto, and rust on the magnets in the flywheel. If so, clean them up with sand-paper, blow out the dust, and recheck the leg gaps, using either a feeler guage or a business card. I prefer the card, because, with the curved flywheel, it is hard to do, if the feeler guage is sort of stiff. The card will allow you to set both side legs of the magneto, at the same time. (works for me!)
Another thing that might help, is to remove the bowl from the carburetor, and clean out any old, gummy gas, that might have been lying in there for a long time.
Disconnecting those switches is not advisable, as most of the time, disconnecting the wires, does the same thing as the switches are designed to do. Shut off power.

I take it that the engine does not even TRY to start, but just spins when the starter is engaged. You need to make the test that mownie suggested. There are probably more safety interlocks than just the seat switch, and one of them could be "shorting" out the system. There is likely one on the clutch/brake pedal, and another on the PTO lever (the one that engages the blades), and one on the shift mechanism (to prevent starting if the transmission is in gear.) Testing these will require a multimeter. There is also a ground wire from the engine to the frame; typically on Husqvarna/AYP/Electrolux/Roper machines, it is a rather small black wire that is attached to the underside of the tractor frame just below the engine and inside the right front wheel. You've likely got a bad magneto or (more likely) either an electrical short caused by chafed insulation, corrosion, or loose connection, or a malfunctioning safety interlock switch. This all based on your being certain that there is no spark at the plug.

Ditto what Mownie said about the kill wire.

Have you actually looked at the spark plug while cranking to see if you have spark?
Just assuming you do or don't can cost a lot of $.

Thanks for the quick reply everyone!
I did replace the magneto with a rust-free one and removed the kill wire. Still I couldn't get a spark while cranking and I was looking at the spark plug..
I ruled out the gummy gas in the carb because it was running just fine before and the gas was new..
I'll try testing out those other parts mentioned with visual inspection and a multimeter. What should I be seeing on the multimeter while testing those parts?
Thanks!

At this point it begs for you to tell how you are checking for spark.
This might sound presumptuous but the metal casing (part that includes threads & tool flats) of the spark plug must be in contact with a clean and unpainted metal surface of the engine while you are cranking to visually check for spark. If you do not have the above detail in place, you are not going to have spark.

May I sheepishly suggest that you may not have put the lever in the start position?

"May I sheepishly suggest that you may not have put the lever in the start position?"

IF it's cranking AND the kill wire is disconnected then any "lever position" would be meaningless!

IF there is no spark and the plug is grounded then either the magneto (armature) or spark plug is bad. (That could include a bad plug wire if it's been gnawed etc. and is shorting.

Great advise above.I don't get by reading if the the rider ran much after the valve work was done. I've seen where locking nut was over tightened on the adjuster, only to run a short time and come apart. This obviously won't create a no spark condition. The other non spark issue could be that the armature is installed flipped. They are stamped either cyl side or this side out.

Well, after a bunch of rechecking, replacing and regapping, I finally got that elusive spark! It turned out that I had TWO bad magnetos (the original and the first replacement, serves me right for getting used ones from ebay, but at least one of them worked! Thanks to you all again for your help, as usual you keep ignorant folk like me humbled.. Thanks!