When writing a Sefer Torah, Tefilin or Mezuza, the Sofer must write the letters with Kavana (intent) for the Kedusha of the writing. The question is whether writing the Tagin (crowns) on the tops of the letters also requires this Kavana. The Poskim write that since even if he would omit the Tagin altogether the writing is kosher, it is not critical to write the Tagin with Kavana. Hacham David adds that nevertheless it is praiseworthy to be Mahmir (strict) and write the Tagin with Kavana.
Based on this assumption that the Tagin are not critical, Rabbi Mazuz suggested a Hidush that if one discovered that one of the Tagin was missing, he may insert it using black ink from a regular pen! He must just be certain that the ink does not contain traces of non-kosher ingredients, since the Torah requires ink which could be consumed.
SUMMARY
It is not critical to write the Tagin with Kavana nor with special ink.
Daily Halacha
The Crown on Top of Sefer Torah Letters- If A Crown is Missing
Lectured by Rabbi Eli Mansour