I have been trying to find a solution for a badly rounded drain plug without success. I have dropped all my oil from the engine by removing the bottom magnetic plate and screen filter, but its not ideal. I bought a set of bolt extractor sockets and tried those- no joy, bought a breaker bar , no joy, new socket with 5 sides (I have been using one already but thought it worth a try) no joy, all I have succeeded in doing is rounding it further. I am going to have one more go on monday with a different bolt extractor socket. If thats not succesfull then its option two.
Option 2: Hunting on the 912bbs forum I found a couple of recommendations that if its really stuck leave it alone as disaster could ensue, go and buy a new sump plate with a drain plug in. http://www.mainelycustombydesign.com/1sump3.html , here is the image below. Looks like a cool piece of kit, robust and a lot stronger and probably flatter than the stock piece of kit.