Keep in mind that it is easier than you might think to destroy your case hitting it with a hammer, so be careful. Grind the bolt flat and flush with the case on both sides. That will enable you to make an accurate center punch in the center of the bolt. Use a smaller drill to bore a hole through the center. It is not that long, so if you are careful, you should be able to drill a hole from both sides that intersect in the middle. If you have to do this with a hand drill, get a friend to help your aim by looking at it 90 degrees away from your viewpoint and helping guide the drill. With care and patience you should be able to keep from drilling through the side of the bolt and into the case. Once you connect the two holes, drill deeper alternating from both sides and you will start to get the two bores to get more centered where they meet. Drill the hole bigger until the wall thickness of the bolt is as thin as you can get it without cutting into the case, but but don't drill all the way through from each side - leave a step in the center. Get a drift or even a bolt the size of the larger hole and grind the end flat. Heat the case very hot with a heat gun or propane torch and drive what's left of the old bolt out. The aluminum will expand considerably more than the steel bolt, but corrosion expanding between the case and the bolt can create a mighty grip. With a little luck, the bored out bolt will be weakened enough to release some of the grip the corrosion has on it. It is important that the step is substantial enough and the drift you use is close fitting and flat ended enough to not expand the bolt internally. Keep heating the case and tapping from both sides and hopefully you will drive out the bolt. Patience is your friend here. If you are not successful, bore the holes larger from both sides until you contact the case, but leave the step and try again to drive out what is left. At this point you can plug one end and fill with penetrating oil and leave overnight and try again. This is worth doing now because the penetrant has access to most of the length of the bolt. With access only at the ends, you could submerge in penetrant for a year and not get very far. If that still doesn't work, you will be left with just boring the whole thing out. If you were careful from the beginning, you should have a pretty good shot at doing minimal damage to the case.