Bought recently this helmet for restoration.
Guys, I swear I will restore this poor kid and bring him to life.

I would definitely tear off the hands of a bastard who painted it in a green paint over the thick layer of putty and fiberglass tape.
You see that I have already deleted the piece of that fiberglass tape and discovered a large break in the shell.

I hope I will find a way to weld it over.

This sounds familiar. A couple of years ago I picked up a relic condition, "partially restored" Russia SSP40 helmet from someone in Russia on eBay for about $5.00. When I got it, it was a similar shade of weird green and had the fiberglass tape covering it and sealing up several holes. I used the thing as my winter project that year, stripped it down to bare metal, filled in the biggest hole and repainted it. The large hole looked like corrosion, but some of the others were clearly battle damage. I am far far from an expert restorer and would not have touched it if it was in any way original condition. It now doesn't look too bad (well maybe from a distance). I wish I had taken a picture of it on arrival as you did. Good luck with your restoration. Please post some more photos as you progress with it.

Regards,
Mikie