I don't know about the KX but I did just that on a friends vintage 400 Husqvarna. The case was really stripped so I drilled and tapped to the next size and used a stainless bolt from the hardware store. I layed the bike on it's side. I'd imagine you could also use a helicoil to retain the stock drain plug.
Go to your auto parts store & tell them what you did. There is a universal drain plug you can tap in. YOU MUST HAVE A SQID PLATE INSTALLED.. I can not remember the name of the plug manufacturer. It does work well & I used that alum. epoxy to further secure it to the bottom of the case. There is a smaller drain plug in the bottom of the universal, so you can drain the oil.