The broken ones you're going to have to drill, and if you don't have precision drilling capability, you will only make matters worse. Take it to a machine shop. If you do have a good precise drill press and the right bits, you might try drilling very small in the center of the broken stud, then gradually larger until you get large enough to insert the smallest size (professional quality!) screw extractor you can find - after soaking with penetrant and heating. But remember this steel-to-aluminum can stick incredibly tight, so you might then end up with more trouble in the form of a broken extractor ("easy out", right, like diarrhea). Unless you're a machinist, don't be penny-wise on this one. Trust me.
If you're taking it to a machine shop anyway, let them do the whole job - including removing the remaining studs (if necessary). You might order the studs in advance and take them in with you, unless your local machine shop has the equivalents in stock (don't count on it).
My two pence/cents/pesos worth.