Might as well do it correctly at this point.

Do you have access to a builder and painter?

Brass or silver solder would take very little effort as Gary mentioned, but it will require some touch up painting.

....or live on the "wild side" with the paint fill idea, but just mask that very small area and clear it with a few coats.

The way I see it is you received a very inexpensive De Rosa frameset that has a few issues.

Sink a few bucks/time into these issues, find the proper decal set and build it.

You'll still have a great riding De Rosa on the cheap if you don't go too wild with the build.