It's probably too late now. The seized ones have already been broken
off and drilled out. I would try tapping on the head to perhaps break
the bolt loose. Also, a shot of something cold to contract the bolt
might be helpful.
Drilling the broken ones out after will be difficult because the drill
will try to wonder into the aluminum. With the right punch in the
center of the broken stud or using a tiny carbide grinder, I would
establish the center of the hole so I could get a straight start with a
drill bit. Then I'd start with a small bit and work up. When it was big
enough for an extractor, I would attempt that carefully. Failing that,
I would proceed drilling to a size I could tap. The risk of losing a
casting to this repair procedure is high.