I used a product called Plastic Weld, I think it is made by Permatex . There are other similar products out there. This is a 2 part epoxy I used it to fill all my misc. holes in my plastic wheelwells and on the bigger holes I just siliconed a plastic piece behind hit and then filled it in with the epoxy. You would never know by looking at it there were any holes.
It would probably fill a scratch the same way but a crack would take more prep work and you would want to add support from the backside. I don't know if it will work in your application but for me it adhered well and sanded down like it was part of the original plastic.
It's best to start out by finding out exactly what kind of plastic you are trying to repair(abs,pp, etc.) sometimes there are letters molded in the part that tell what kind it is. And then make sure the product you buy is compatible.