Best to post the Sears 917.xxxxxx number so one can determine the actual engine.
22.5 HP Kohler horizontal doesn't make sense per my database.

engine is a kohler CV22S-67527
machine is 917250560

A CV22S-67527 is a VERTICAL V Twin. NOT Horizontal!
A 917250560 is listed as having a CV22S-67515, a slight difference.

I suspect you may have blown a head gasket.
Best way to check the spark plugs is with an actual spark tester.

You can typically see a blown head gasket by removing the valve cover on the suspect cylinder.
Start the engine (have a rag handy) and look for a "mist" blowing out in the gasket area on the push rod side.
Also examine push rods for excess looseness, bent etc.

Although Bill may be right about the head gasket, I'd suggest you check ignition first, as the path of least resistance.

I assume you troubleshot the dead cylinder to determine which is bad, by either shorting or disconnecting each plug individually. Once you know which cylinder is not firing, you can remove and reverse the two ignition coils, and see if the problem migrates to the other cylinder. The coils are identical, p/n 24-584-03. You've already had the coils off, so that isn't much work.

I doubt if the speed control module is the problem, because it sends the same signal to each coil. I wouldn't rush out to buy new coils, because they are about $60 for genuine Kohler, and $10 to $15 less for aftermarket.

If you still have the same symptoms, then I believe Bill is right, and off come the two cylinder heads.

Bill is also correct about the CV-22 being a vertical crankshaft Vee Twin.
It is the orientation of the crankshaft that determines whether the engine is considered vertical or horizontal. Not the cylinders.
CV-22= Command, Vertical crankshaft, 22 horsepower
CH-22= Command, Horizontal crankshaft, 22 horsepower.
I posted this to give a clue about how the engines are titled because for some components, it really does make a difference whether the engine is configured as vertical versus horizontal.
ggoyeneche is also correct in his suggestion that swapping the ignition coil units to opposite cylinders will determine without a doubt whether the problem is ignition or otherwise.
Bill's offer that only a spark tester is adequate to check the real "firing power" of a coil/magneto is true.
The biggest reason for this is that the effects of engine combustion chamber pressure (aka compression) is profound on the ability of an ignition coil to generate a strong spark.
Any method of testing the spark of an ignition coil or magneto that DOES NOT take into account the effects of compression will be inconclusive. Or in other words, it might make a spark that will jump a gap or flash a light out in the open air, but under compression, it will not spark.
I don't know what type of test light you are using to check for spark, but unless it is an inductive test light that can be clipped over the spark plug cable with the cable still connected to the spark plug while the plug is properly in the engine, that method too is subject to giving iffy results.

ok the vert/horiz was my mistake. it is vertical shaft. also the plug wont spark when grounding it and spinning the starter. the other side will. head gasket is a no because its not just that it isnt firing, as i said it wont arc when grounded. im just using a cheap 12v test light which i knew wouldnt give me a conclusive answer unless it showed nothing. it lights up with the rotation of the engine whether i test the spark plug installed in the wire or just test inside the boot. i didnt try switching the coils to see what happened, but i will. mownie you did answer my question when you said the coil can give a weak spark that wont be strong enough to fire while still activating the test light. that was my area of inexperience, that i didnt know if they are prone to weakening or just commonly just fail all at once. all of you have given useful insight, thank you. i will try switching coils and i will repost with the results.

swapped coils today. problem switched sides so that answers my question. thanks everybody.