Greetings,
I having looked at the photos of your wiring from which I think that somebody has been messing about with your bike before they sold it on.
'Left hand' or 'anticlockwise' threads are normally only fitted where there is a risk of the fastening self loosening itself through rotational forces, that is why on older completion style vehicles directional 'knock on' nuts where used. Following is a picture of a Jaguar hub nut;
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I rather suspect that somebody might have had the crankcases apart and incorrectly rethreaded the drain hole from the inside of the case because left hand thread cutting taps and bolts are not so easy to obtain.
I think that I would be tempted to;
a) clean up and then drill the sides of the bolt so that you can lock wire it into position
b) replace the crush washer with a new one
c) refit the bolt but carefully do not over tighten it especially if the threads look in anyway damaged, all you want to do is seal the hole to prevent oil seeping out
d) lock wire the bolt to both prevent it from loosening and to remind you that it has a non standard thread
Alternatively, you could have a thread specialist rethread the drain hole and fit a new bolt and washer for you.
Additionally, I would replace the oil, the oil filter and its two seals in the side casing and most importantly inspect and clean the oil filter which is positioned in the frame front down tube.
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It sounds as though you should get this bike checked out by somebody more experienced with similar machines, perhaps a Suzuki dealership, to make sure that there are no safety issues with it.
Hope these comments help.
TTFN
Hugh.