If the broken bolt is not broken off more that a couple of threads below the surface I would do the following:
I would install a sharp 1mm tungsten in to the tig torch and obtain some E70S2 .023 to .035" diameter filler rod. If you do not have that diameter tig filler rod, use some wire from a mig welder for tig filler rod.
Machine a 30 to 45 deg cone shaped into an unplated 6mm nut. When machining the cone shape into the nut, do not cut the cone into the last thread or two. We want the small end of the nut to stay at around 6mm.
Start welding in the center of the broken off bolt, building up material on the end of the broken off bolt until you have a 4 to 5 mm shaft protruding about 1 to 2 mm above the head surface.
Place the nut over the 4 to 5mm shaft with the large end of cone in the nut facing toward you. Weld the nut to the 4 to 5mm shaft. Wait about 1/2 minute after the red disappears from the nut to gently start unscrewing the nut while using penetrating oil. When unscrewing it will probably be necessary to rotate the screw in and out by using gentle 1 /16 rotational increments to allow the penetrating oil to work in and prevent the threads of the broken bolt from galding to the aluminum head.
It may be necessary to run a tap through the threads to clean up the top few threads and machine a new gasket sealing surface if you are not a seasoned tig welder.