I Have had this problem. I used an oxy to heat up the bolt and then they turn out but you need to be pacient and turn it out turn it in, turn it out and in for a few times because you should have rust right up the bolt. With the ones that are broken off. Use some sort of jack or hydrolic lift to move the head up evenly. You can damage the head more by twisting it or lifting it unevenly.
My first time I did it I used a bottle jack on one side move it a tiny amount, release the pressure. Apply spacers on the other side. Jack it a little more. Dont do to much though because you are going to extrude the aliminium head and it will make a lip around the hole and this isnt good. Lubricants or hydrolic oils are good to use. Find out where the bolt has broken off though because they step down part way down the bolt and this can cause damage when taking the head off. If needed drill a little. Then heat the bolts up and use multy grips to turn them out while still hot.