First get your self some proper penetrating fluid such as PlusGas, WD40 is not what you need for this.
Depending on your skill level you could weld a nut onto the remains of the bolts, then once the PlusGas has been applied a few times, use heat to free them up.
Alternatively google Easy Out bolt extractors.
If you just try to drill them out you will more than likely damage the thread and then would have to use Helicoils or threaded inserts to repair.