San Hill - I suggested to get a couple of similar length bolts (from hex head to end of thread) because 1 only stud can be easily bypassed. The stud sleeve only holds the plug wiring, correct? And please post the bolt thread specs so others will know.
We have all been there with broken bolts. You can either have a professional remove it (very expensive) or do it yourself. If the latter, roll-up your sleeves.
Someone else may have a better method and will likely chime in. The following is how I would would try to remove it.
Now on to the broken bolt. Fill all the holes/spaces in the heads with rags. Soak the bolt hole with penetrating oil. Do this for a day or 2. At some point, you will need to completely cover all exposed parts of the valve train, with only the bolt hole exposed. A shop vac and or small magnets may be needed to capture cuttings. Possible a 3rd hand.
You will need the following tools. A sharp centre punch and hammer, small drill bits (1/16, 1/8, 3/16, buy several of each) or similar size, a 1/4 or 3/8 reversing drill. Preferably 2 speed. You will need to drill at a very low speed. And left hand drill bits could be very helpful.
You will also need small extractors and a tap of the thread size. You will need to clean/repair the threads.
Once exposed area's are completely covered, try to spot punch the centre on the bolt. This is critical, will be difficult because of the uneven shear of the bolt. Start with the smallest bit, drill with very light pressure and at a very low speed. Weight of the drill only with a sharp bit and cutting fluid should start the cutting. Drill about 3/8 " deep. Then the next larger bit and alittle deeper, then the next size drill bit and deeper yet.
It the drill bit flexes or wobbles, your are applying too much pressure, If it charters, either you apply more cutting fluid or change to a new bit.
If the drill bit breaks, your fudged. Use very light pressure on the drill.
What you are trying to do, is reduce the broken bolt to a shell. At this point it will easily come out.
