Yes, It's a 2 3 4cyl, I should have mentioned that. As for the bolt/stud he broke, It's the one that is still installed on. The head in that diagram, lol. The stud that the tensspring pushes against. I found this pic online and added an arrow to it.
Ok, so you have room to work. Getting broken bolts out is a little bit of a art. You just have to be accurate and careful.
You have to assume the worst when starting out, and that the bolt is froze solid in the engine. The best scenario is it just fatigued and broke, and is just sitting in the engine.
So to start you need a good center punch. You need to carefully lay the center punch as close as you can to the center and make a mark in the middle of the broken bolt. Get back and stare at it, if it's not in the center try again. You can put the punch right beside a previous punch mark and punch it, or angle the punch a little to push the metal over if you are half a punch mark off. it's very important to start in the middle of the broken bolt.
Then get a small drill bit. 1/8 is about right. Start drilling, you must be careful here too and keep it in the middle. This drill bit is small, so when you start out if you need to steer it a little bit to keep it in the middle you can. Until it starts getting deeper and then of course you might break it off. But this is going to be your guide hole, try to keep it in the middle.
Then get what they call "easy outs". They come in a set. If your 1/8 hole looks really good, you can open it up a little bit so one of the larger easy outs will bite, and try to back it out. If the stud is just laying in the block, it should come right out.
If it's really stuck, this is what you planned on from the beginning, you will have to open the hole up like you were going to tap the threads, because you are going to tap the threads. This is why being in the center is so important. Of course you will need to figure out what thread it was, I think this little engine was always metric but make sure.
If you ended up being a hair off center, it doesn't usually matter too much. If it gets really ugly you can use a stud and jb weld it in place with what threads are left.