Hi Pierluigi and Cedric!
Good to see that my posting has inspired others.
#1:
I used an L-shaped drill...

NO I didn’t!
I used the other sprocket as template and marked the bolt holes position and drilled the holes from the outside.
Then made the countersinking with this Dremel bit, working as vertical as the sprocket permitted...:
Superglue the bolts, cut the ends + epoxy putty.
#2:
As the studs diameter grows when I made them "my way of" castle nuts so the countersunk is made 5,5mm.
#3
This is a tricky one as the sprocket s are now painted an I cant see how much I added to the triangle shaped hole.
But here is some measure that might be of help for you.

Please note the comparison with a real Panther sprocket.
Not perfect but a good bit better as the measure 14,5mm is made far too small in the kit.
Kind regards
Kent