Wow....... that hole must have got wal-erd out bad.......!
There are other ways to fix that thread in the case but it takes the right stuff and the know how, I have included a link here that shows how simple it actually is if you have the tools, a simple propane torch, a power drill of some type, a small Stainless steel brush, a small can of acetone or just a can of carb cleaner, some aluminum brazing rod and asmall grinder or just a simple Bastard file, correct drill bit and M6x1.00 thread tap and you can make that thread good as new and actually stronger than the aluminum itself. I've used the stuff and it the aluminum is CLEAN.... and its used right it works great.
By the way you dont need to have a gas grill to heat the engine, that would be a disaster and blow the grill sky high probably.....LOL! just get inside a garage or work shop or in a place out of the wind so you can see what you are doing and there isn't air or wind blowing against the aluminum you're working on because aluminum cools quickly and you have to maintain the temp for the brazing rod to flow correctly and bond right.
As Huffy D said, you will be taring down the engine completely if you replace the case, it isn't hard to do if you are fairly mechanical minded and you have the right tools for the job.
vids that show most of what you will encounter with a tare down.
Hope this helps