I had trouble with my 25 hp. Kohler not running well when it got hot.  The hotter the day the worse it was.  When it started acting up the choke would help it run but it didn't have much power.  I tried all the easy stuff to no avail.  Finally I changed the ignition coils and that solved the problem.  It seemed to have 50% more power.  I found out that the coils were a common problem.  Kohler advised to replace them both when doing so.  They had a new part number that replaced the coils susceptible failure.  You may have spark but when it's hot and under load it just isn't enough.

Try the easy stuff first but replace the coils if that doesn't work.