Before you worry about a rebuild do some maintenance. Add Marvel Mystery Oil (MMO) to the gas( 1/2 oz per gal) and put a few hours on it. Then look at: an oil change, air filter, ignition, valve lash and breather. If any are questionable change it. Then, after a couple of weeks of running it with MMO in the gas, check for smoking. Most times it will have cleared up.
If it still smokes a little you can probably run it for a lot of hours by just adding MMO to the gas. When you get to the point that it smokes too much a rebuild is the best choice. The Kohler is a heavy duty engine that would be hard to match. I figure about $175 for the parts and do the work myself. Good Luck
Come to think of it check the breather first.
Edited by boyscout862, January 23, 2013 - 03:41 PM.