On the next day I figured that I could actually make a special bolt instead, which is BETTER THAN ORIGINAL, because the hole it goes into in the engine happens to allow for a bit of extra length: I checked: 5mm longer is good. That way I can use even part of the old unused thread, which is utterly intact even after removing the stuck bit. So I removed the 8mm x 40mm bolt from the "sleeve" and replaced it with a 8mm x 45mm bolt - i.e. now I have a special bolt, which is 5mm longer than original.