Lonnie,
My answer is; yes, no, maybe. A lot depends on the type of plastic used in the covers. Some plastics are notorious for being unable to bond to. My suggestion would be to visit a good local autobody shop and ask them what they use for repairing plastic auto parts. Also try searching you-tube for videos in repairing plastic parts on autos. (I suspect you'd get more results than a similar search on outboard motor cowlings.
That said, I have successfully glued and bonded to some plastics with "standard" epoxy resins. Just repaired a friends broken transducer housing.
Yet, I have a rotomolded kayak which required repair by adding melted plastic into the damaged area, much the same as adding filler rod under the heat of an acetylene torch.
If you find that epoxy will in fact adhere, I'd suggest drilling a hole at the ends of the cracks to prevent further cracking along the same fault line.