How to find a short circuit (bad ground) on my renewed Cosa2 125


I am finishing a good refreshment on a cosa2 125 for my wife. This Cosa wasn't starting with e-start, cause there were some electrical problems, said the previous owner. Right now, with battery loaded, as soon as I put the contact on, the fuse is blowned: there is a "bad" ground somewhere. But where ???

Do you have any good advice to find the bad ground ? Where to start...

Another question is about the stater motor: is there continuity beween + connection and the ground ?

I have and I am not sure this is OK !!!

Thank you by advance for some help !!