Depending on just how much is delaminating and how serious the other cracks are, you can use a bit of ABS and MEK (methyl ethyl ketone) to make a paste to fill in the cracks. MEK is just a solvent, so you are really just making an ABS cement.
After that, many folks have reported success using truck bed liner in several coats to cover the entire roof and restore a water proof surface that seems to hold the rest of the ABS in check. Some have gone so far as to completely remove the ABS shell, apply a primer to the bare foam and then spray/roll coatings such as bed liner on it. Bed liner seems to be a clear winner for repair versus full-out roof rebuilding and replacing.