Would you use
Isaac itself on a girl? If not, I don't see how changing the spelling/nationality makes it much better. Especially considering that most people won't know the etymology of
Isak, I think it is likely to seem like a kre8tyf spelling of
Isak to feminize it. The way people think adding a Y makes
Aidan (Aidyn) feminine or something. It doesn't.
Besides, I love
Isaac itself for a boy; why mess with an already good thing?