Nonsense. There can be several reasons for the backfire
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
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.