The Kohler Command engines have been around a long time. At least 20 years. I think of them as one of the toughest air cooled engines you can get for power equipment these days. I have seen several with well over 2000 hours on them and still going.
Like all engines, they do require regular maintenance. Most valve trouble I have seen is due to either debris packed in the fins or escessively lean fuel mixture.
You can't get the smaller single cylinder Command engines any more. I think 18 hp. is about the smallest offered.