Stop dril the cracks with a 1/16 bit Then sand to clean and fill with some epoxy and then again sand to smooth. Only real way is to redo the entire cab over to make pretty, but for temp repairs the above is what is needed. As to the crinkles that is usually due to weather effects and again needs to be sanded smooth and reepoxied/fiberglassed. painting is optional but needed if you want to pretty the cap.
yes you can rout the cracks a little for the epoxy/fiberglass rosin setup but really after all is said and done the whole cap will probably need rework. The stop drill is at the end of the crack and keeps it from spreading further. Also do the same for any metal cracks you ever find.