Is it permissible for a girl whose hymen has been 80% damaged due to an unintentional accident, with no intercourse involved, to undergo an operation to repair the damage? And is it permissible for her to conceal what happened or claim it was an accident when she gets married?

What happened to you with this girl is from the deceptions of Satan. It is obligatory to repent and be cautious in the future. It is not permissible to repair the hymen. There is no need to inform the suitor about its absence. If her husband discovers the absence of her hymen, she should not tell him the truth of what happened. Instead, she should tell him that she did not commit fornication that necessitates a Hadd punishment.