That's unfortunate!
Sorry, I don't have the size or a picture available.
I've had a look at the parts catalogues (both Beta or Suzuki), they both have no indication about the actual size(s).
You will need the following Beta parts:
1170511 000 Bolt
2778601 000 Washer
This should be available by any Beta dealer, but I doubt they have them locally in stock.
Shipping from Italy (to germany) is usually about a week, with Corona maybe a bit longer.
You may get these parts faster from your local Suzuki dealer, remember the Alp 200 has the same motor as the Suzuki DR 200 and I doubt the bolt/washer changed a lot over the years. If you need the Suzuki part number, I can try to find them out.
Thanks to a tip from "halpgas": the torque for this bolt is only 10 Nm, so it's a very good idea to put some "Loctite medium" on it! Seems his tip is becoming even more valuable ...
Hope this helps a bit!