Let's say the Torah parchment has a hole such that a letter is missing where the hole is, making the entire Torah "pasul".
May one use either of these methods to fix the problem?
1 - Get a small blank piece of parchment; glue it to the back of the original parchment, and write the missing letter after it has been glued?
2 - Get a piece of parchment, write the letter on the small piece of parchment, then glue that to the large parchment such that the letter appears through the hole in the correct place?
If neither method is allowable, does this mean that the only way is to rewrite the entire yeri'ah (parchment sheet)?
Note: I am looking for a halachic source. Additionally, you may include information of the practical aspects of why this may or may not be a good solution (from a safrut or preservation view.)