Jenny and I would give you our Lawyer response ... it depends. How big of a piece are you talking about, would it be a visable issue if it were to be repaired, if it was repaired, woulld it frustrate you every time you saw it? Those are some of the points I ask myself on any part I'm restoring on our Eliminator. Another good one to ask ... if I fix it, will it stay fixed or just break again?
On plastic parts, you can try glue or epoxy. My experience says epoxy works best. Does it need to be braced to prevent it from breaking again and if I want to brace it with another piece of plastic on the back side, will I have clearance on the backside when I go to reinstall it. Before I try the repair, I'll see if I can mock up the repair by taping it together so I can test fit it ... if I have to add some additional structure on the back side. If I need to try a different approach so be it ... at least I don't have glue all over.
When Jenny and I took apart her fuel door cover, the reflector came out like a jigsaw puzzle. I used super glue to reassemble it (all the broken bits were behind the mounting plates) and then I reinforced the reflector on the back side to give it some additional strength. Without seeing the broken part ... it's really kinda hard to advise, but remember ... some glues bond by melting plastic together chemically. That can change the shape and appearance of the part. Plastic modeling cement is an example of this. By not using the right material, you can do more damage than you already have.