When selecting piston kits make sure you select the correct design on all K Series and Magnum engines.
There are 2 basic designs, the Old Style piston and the New Style (Mahle) piston. If you have the Old Style piston and you are Not replacing the rod you have to stay with the Old Style piston design.
If you want to upgrade to the better New Style (Mahle) piston you can but you have to also replace the rod with the New Style rod.
The Mahle style piston skirt profile was modified to provide a better fit within the cylinder bore. The ring channels were redesigned and the wrist pin was strengthened. A different connecting rod is required because of the extensive design changes.
Beware of shops selling the aftermarket "rebuild kits" more then likely these are imports. These parts in many cases will not inter-change with genuine Kohler parts, example aftermarket rings will not fit in genuine Kohler pistons. They are also selling the old style piston design, further they are also claiming "It's what the rebuilders use". In my 30 plus years in the power equipment industry I have never heard of the term "rebuilders" in the small engine business unless they may be referring to backyard mechanics that are using aftermarket internal engine parts. You may be saving in price but not Quality.