If there is any thread sticking out first thing to do is hit it a few times as sqaure as possible to try and relieve some of the corosion that it has on its thread , soke it with wd40 overnight if posible and then very gently try removing it , but try tightening it slightly first and working it back and forth constantly , if no thread is visible then very carefully centre punch it then drill out , firstly to say 3mm and keep as square as possible , as this will aid the cause if you need to drill it out totally , try and drill all the way through the bolt if possible then soak again with wd40 or similar , at this point very carefully try using an easy out and see how siezed it is , if still solid the only way is to try and drill out and the helicoil it , some good garages have helicoils or engine builders ..... I have a full set cos of my past , but live a bit far away