Looks like an oil seep maybe from the valve cover gasket. Or dirty coolant could do it too.
Clean it off well and ride it to see the source.
A trick is to throw baby powder on the area and look for the stain as soon as it comes out.
Hmmm... perhaps you should speak with a urologist?!
Seriously, is probably just a cracked/dried-out valve cover gasket.
Adding to swifty's advice, I'd clean everything off as good as possible, finishing with a good spray-off with brake parts cleaner to dissolve any/all traces of oil.
Let it sit for several hours and then give it a light dusting of talcum powder.
Blow it off with some compressed air and any lingering powder should reveal the source of the leak.
If no leak, then start her up and let her run several minutes. Shut her down and repeat again with the talcum powder.
Let us know what you find!
i think maybe is comming from the sparks tube inside,
Frequently, the heat may cause a little hole and there is where the leak comming from.
I remember see a few of sleeve plugs with tiny holes or degradation cause the years with that problem in many bikes from 80-90 years. So i will take a look there with a light or just even better if you can extract the sleeve plug and examinate it.