Depending upon the crack, I imagine you coul repair it. If it is a small crack, a good quality glue. But for best results, some type of a backing on the underside to strengthen things up.
If it is a larger crack, then one of those fiberglass repair kits (get at auto parts stores), would work. That would require more work in sanding and painting, etc, but would be stronger than just glue.
Not surre what type of plastic is used for the roof, but that would be important to know so you can select the right type of glue.
your roof is most likely an ABS type plastic. You will need a glue that will "attack" or soften the surfaces and fuse them together. Jump on the internet or go to a hobby shop, they should have an ABS glue as most plastic model kits are made of this or a similar type plastic. A good shop should also have some ABS sheet stock to make backer strips to go behind the cracks to reinforce and increase the glue surface area. Rough up all the surfaces with sand paper to get a good mechanical bond as well and it would be good if you could clamp or weight the repairs to get a good bond. Sand down the excess glue in the seams and Krylon the roof and you should be good to go. Hope this helps.