The source code for Binding of Issac is not available. Rebirth is already on the 3DS e-shop. The series creator said Afterbirth would not be ported because it raised the system requirements too much for the 3DS and Vita.
Yea now that I've done some more research I am thinking that making my own binding of isaac type game would be easier haha.
Sad, I really hoped for an afterbirth port since that one unlocks so much more things. Rebirth is fun tho!