Nonsense. There can be several reasons for the backfire problem.
Excess carbon buildup in the combustion chamber can cause a hot spot that will ignite the fuel mixture after the ignition coil is shut off. A hot spot inside the muffler can also cause the unburned mixture to fire.
Next time you use the tractor, do not shut it off immediately. Instead, bring the engine speed down to an idle and wait for at least a minute or two before turning the key off. This will allow the hot spots to cool down.
I also suggest that you make sure you are running the correct heat range of spark plug with the proper gap. Set the points to 18 thou. Be sure that your air filter is not clogged and that the fuel mixture screw is set so that there is no black smoke in the exhaust stream.
The valve lash should also be checked. Go to Kohler dot com and get the Owner's Manual and Service Manual for the K-series engines. They are free downloads. Your engine has ACR which is Automatic Compression Release. You cannot check the compression on engines with ACR without defeating it first because it holds open a valve to make spinning the engine while starting it much easier.
Focus on bringing everything into spec first before condemning an engine that may well have many years of service left in it.