If there is enough bolt protruding from the head, I prefer the double-nut method (as in: jam nut). Penetrant, gentle heat, a little in, a little out.... More soaking, heat, back and forth. Zen movements. Be the bolt.

I'm not sure the valve cover is the best choice for a drill guide. I have flat filed/ground the top of a broken bolt. Using a bushing or something similar held with vise-grips, drilled just enough to create a little cone to center a smaller bit to start an extractor bit. But my favorite is the nuts-as-bolt-head method, if you can.

Heat will help a lot. Aluminum expands more than steel with heat so putting a torch on the head around the bolt will help the PB Blaster/Liquid Wrench or whatever penetrate. It will also open up the hole, reducing the grip on the broken bolt. You don't have to heat it to red hot or anywhere near where you'll risk warping the head. Heat it until the penetrant smokes, then try to twist it out.

thanks but the bolt is broken flush to the head. thinking that any drilling I may use a left hand bit. I do not think it is seazed.