OK, the head bolt threads on your engine are 1/4-20 UNC. Do you have access to a 1/4-20 tap? You may need to purchase a few 1/4-20 "helicoil" kits as well. These items can be easily found online if they are not available locally for you. The "helicoil kits" include the appropriate sized drill bit, one plastic helicoil insert driver/insert tool, and several 1/4-20 helicoil inserts.
But, before we get to the helicoils, let me ask a few questions. Are the head bolts broken off even with the cylinder block deck? You may be able to avoid drilling out the broken stud if some of the broken bolt shank is sticking up from the block. Clamp the broken bolt stud with vice grips while heating the block/stud, once enough heat is applied, you may be able to gently rock the broken stud out with vice grips. Don’t force it, be patient, the broken stud will start to "rock back and forth" very slightly once enough heat is applied.
If the bolts/shanks/studs are broken off even with the block, they must be drilled out. This is not an easy process, and success will depend on your skill level along with the quality of your drill bits. You may want to consider letting a machine shop drill out the broken studs if you have had little/no experience doing this type of thing.
Post some pictures of your broken bolts here, that may help us find the best solution for you.