The Kohler manual tells you that you can drain the unit by opening the drain valve at the bottom of the radiator. This will drain about two gallons from the radiator (if full), but will leave about one gallon in the block. There appears to be no drain plug in the block, nor is there any way that I’ve found to snake a hose into the block to siphon it out.
According to the tech support people I talked to, if you are just doing routine maintenance changing the coolant and it’s still in pretty good shape, just add two gallons of fresh, diluted anti-freeze to the radiator and be done with it. If, like me, you ran radiator flush through the system before reading this, you will need to purge that from the system. You can do this by removing the lower hose from the radiator and running clean water through the block. A female hose end just fits inside the coolant hose to the block. You can even tighten the hose clamp for a better fit. Run about 10-15 gallons of water through the block to purge whatever is in there. Tap water is OK for this purpose. But now you will need to purge that with clear (de-ionized or distilled) water. I used a submersible pump in a bucket with clear water. You might be able to get away with lifting the hose end above the generator and pouring water into the hose using a funnel. You should run at least 3-4 gallons of clear water through the block at this stage.