Re: Oh no! Now what? Starter bolt hole cracked.
I had a similar broken out hole on a cast iron head and the machine shop was able to weld it back together, worked just fine. I don't know If I would do it for a starter bolt though, probably better to just find a new block. If the welding works that would be great, but it would really suck to get stuck out on the water and then have to tear out the engine again, all to save a couple hundred bucks.
On JB welding a stud into the block. I don't think you will be able to get the starter mounted back on the engine, with a stud in the way. You would have to put the starter on the stud 90* off to the side and swing the starter into position, probably not enough room or obstructions that will stop you. You could mount the starter while the engine is out, but then you would be hosed getting it back out again, in the future, with the engine in the boat.