If it were my bike, I would apply several soakings of a liquid penetrant. A 50/50 blend of acetone and automatic transmission fluid (ATF) works well, but there are other products available at auto parts stores like NAPA. After a few days of application and soaking I would use a manual impact driver, the kind you whack with a hammer, to deliver a shock wave and a slight twist.  If that did not work, I would use a torch to heat up the part of the casting where the plug resides. If you have to resort to that kind of operation after using a penetrant, you can expect a bit of a flame up from residual penetrating fluid.  Manual impact drivers can be set to twist in both directions, so be careful about that. You wouldn't want to drive the fill plug in even tighter. 

best of luck 

@8053 Thanks James.