BarelyThere, you want to marry the guy that you were disfellowshipped for having sex with.
Presumably you and this fellow are now living a "chaste" and "approved" life, which means no sex for either of you and no "mutual sensual gratification."
Was he disfellowshipped for having sex with you, or for a different reason?
What if the two of you went to the elders and said you want to marry and you need to know how to get this done.
Scripturally they CANNOT forbid you to marry, but they can determine whether you are allowed to be married in the Kingdom Hall and they will determine how long you will have to wait for reinstatement.
...and your fiancee is OK with his plan to fade, right? RIGHT?
If he's not, you'll be going from the frying pan into the fire.