mercury casts an alloy in all there aluminum blocks so this problem never happens as aluminum corrodes and seizes any bolt so you were lucky to have only one broken - the best way for a pro repair, grind the top of the stud (broken bolt) so its pretty flat, center punch the center , using a small center drill drill the center not all the way , using smaller drill bits drill through the stud and no further you will feel it when it drills through the stud , next get the tap drill and drill through the stud, using a countersink or larger drill bit just use it to chamfer the radius of the new hole, basically just cleaning up the top of the hole then use a 3 flute taper tap same as your head bolts use some cool tool or cutting oil to aid in tapping and just a bit at a time in and out clearing away any chips ,i say use a 3 flute as the 4 flute taps bind up and may break , i cant stress enough take your time and if you dont have a drill press drill straight not crooked, dont weld anything to the existing stud as you may get into a bigger mess, when the tap starts to tighten up back it out air blow the hole to clean out metal chips and proceed until your through and done! a pro job!!