Replacing kickstarter seal

You don't need to remove the bush to replace an o ring with x ring. (My o ring and x ring are smaller than the recess. I don't know why.  But I part filled the groove with silicone sealant first..and let it set after re-assembly)

Surely the key should be imperial? Most likely 3/16". To help you get out of your dilemma, may I suggest a 'bodge' of hammering a torx bit into the mullered head and try undoing it that way? Usually pretty successful, but the torx bit will only be good for the bin afterwards.  Failing that, get hold of some 6mm 'left hand' drills (yes, they do exist) and drill the head off? Quite often it will start to undo long before the head comes away. I'm sure there will be more suggestions soon, George   

Mine has socket head screws. Definitely not metric.  Get the right one...it might fit deep enough to move.  Allen keys should be snug...even tight...they are high precision 

1/4" - 26 TPI X 1 1/16" UNDER HEAD SOCKET ALLEN SCREW BSF in effect with a 3/16" socket. You will have nothing to lose by using red Loctite to take up the slack in the bush, even the green would do.

Graeme, Now you have identified the correct screws. The ones with 3/16 sockets. Try screwing these into all the gearbox casing tapped holes.  Just as the metric 6mm screws deformed the thread, the correct screws will probably deform the threads back to correct. You will need the Allen key.  Using a tap to recut the threads can sometimes leave you with a very sloppy threaded hole, because of the aluminium you have taken away.  Peter ​​​​​​ ​​​​​

One way around this dilemma is to turn and tap a grooved screw preferably a new one,  buy one more than you need. A Dremel cutting disc is used to cut a groove along the threaded section 180° apart on a slow spiral if you can. Enter this bolt as far by hand as it wants to travel, once it "sticks"  a slight tap/taps with a toffee hammer will help it along. Continue until the bolt bottoms out, then onto the next bruised threads. Failing all of this advice within this forum rethread inserts are the best way to restore these fixing holes  Using the method described above you may find the use of "Loctite" red or green to take up the slack should you fel the need.  

M6 standard thread has 1mm pitch.  That is 25.4 threads per inch.  Because it is a smaller diameter (6mm) than 1/4" (6.35mm), it is not surprising that it can be forced to fit.  A proper 1/4" x 26 this might well just go back in.  It's not very highly loaded 

Please people the colour of loctite isn't as important as the number! - the loctite website will guide you! Dan