That plastic is 40 years old. It is ABS and has more that likely degraded badly. If it has yellowed, it has likely lost much of its ductility. If you are going to use the unit going down the bumpy US highways, it will likely crack again, in the same locations. They units move around as they go down the road and stress the interior skins. It is messy, but a thin coating of fiberglass is the easiest, least costly, way of extending the life of the unit. I have never tested whether a coat of elastomeric texture paint might work, if you can find one to stick to the ABS.