Best solution is to drive it (with the leak) to a workshop that has a Helicoil kit. This is a box of various specially-sized drill bits, taps (for cutting special threads) and items that look like coil-bound springs. The 'springs' are actually hardened thread inserts that a special tool winds in to the specially-cut larger thread in the alloy. Then the operator snaps the tang off the end of the thread insert and the area is dressed with a die grinder to leave a perfect, repaired thread (potentially stronger than the original).
The last time I watched this done was for a tensioner bolt in an alloy engine block.
It wouldn't be worth purchasing the kit for a one-off operation, but it is probably worth paying the modest cost for someone to sort it out properly for you.
There are three other DIY options.
Option 1 (Good, if it works out) - obtain an M7 tap (for cutting a 7mm thread). I assume we're talking about a FIRE engine, in which case I think it was originally an M6 thread (10mm-across-flats bolt head). From where you buy the tap from, find out the drill size (probably an odd size such as 6.5mm) and get this drill. Then, drill the hole out and use the tap to cut a thread. Only do this once you've got hold of a suitable M7 bolt - 11mm head - (perhaps two!) - a hard size to find here in NZ, potentially easier in Europe-friendly England. Be aware that if the drill wobbles, the thread may end up a bit loose and therefore weaker than the original.
Option 2 (just OK) - use slightly longer bolts to reach some of the undamaged thread further into the cylinder head. It can be difficult to find bolts exactly the right length. Be careful not to crack the cylinder head after the bolt bottoms-out (use washers).
Option 3 (not good but might get you out of a pinch) - use Loctite Master Gasket 518 - a red translucent goo that sets very hard when there is no air present between materials (NOT silicone sealant). Ditch the gasket and literally 'glue' the housing onto the cylinder head. Coat the bolt with epoxy resin (e.g. Araldite) and put in as far as you can. Leave 24 hours to set.
-Alex
