I suggest removing the oil filler tube by unscrewung it, and plug the hole to avoid dropping bits and muck into the crankcase. Use a right angle drill to drill out the broken bit, trying to ensure that it is properly centred and in line with the bolt axis. Gradually increase the bit size until you can clean out the hole to size with 5/16" taps. Be very care ful not to impinge on the cylinder outside wall. Assuming all this works (it has for me in the past), use a 5/16" stainless steel stud in place of the bolt,loctited in place, make sure it has the right projection through the water joint, and fix the joint using 5/16" brass or stainless dome nuts on fibre washers; this way there should be no need for sealant on the joint face or on the studs. I'm a bit far away to lend you my drill, but a local tool hire company should have one quite cheaply.

Location: Just north of Cambridge