Re: Blown spark plug hole- is it fixable or junk?
There are two issues with a welded repair. The fill metal (welding rod) used won't be the same alloy as the base metal (engine) and it may result in an area weaker or stronger than the area around it. It may also expand and contract at a different rate. Also the welded area would need to be baked (heat treated) for stress relief. That means taking it off and disassembling it. It may also warp in the oven and need to be remachined to mate to the rest of the case. If you are doing all that work, might be better to just replace it.
In aviation field repairs are always done mechanically with rivits, adhesive, patches, doublers, and inserts just because it is impractical to get bigger pieces off the plane and into a welding shop.
There is a type of helicoil called a twinsert, which is a coil inside a coil, for fixing bigger damage. There is a also an oversize Time-sert called a Big-sert. Time-serts and Keenserts are just as good. It does boil down to personal preference which to use. I also recall an insert they used to sell at the autopart store that cut it's own hole, and just stayed in place as the repair. It does increase the height of the sparkplug, and that is usually an issue on an inline outboard. But cheaper, and doesn't need special tools.
--Jim