The old Yammy tank was rusting on the inside so we cleaned it with vinegar, works great, thanks for the tip BC :)
We decided how wide the new tank should be. We're adding 2.5 cm extra in the back and 9 cm in the front so it narrows in the back now. The fuel cap is off center, on the right side, so we can fill it up to the max when it's standing on it's side stand...!
For the headlight we found out that diameter 15 or 16 cm would be perfect.