Test the plastic with a soldering iron. If the iron melts both pieces of plastic, you can try plastic welding the 2 pieces back together.
If the plastic doesn't melt, that means it's thermo set plastic. To repair it, I would take the whole assembly off the car. Using layup epoxy and Kevlar cloth, which will make the strongest repair, use a bolt a little smaller than the broken piece. From the back and the front make repair patches using the epoxy and Kevlar cloth. Of course the plastic piece should be roughed up with 60 grit sand paper and cleaned. When the repair cures, the bolt will be sticking out where the original plastic stop was. Coat the bolt with epoxy, a really thick epoxy like jb weld will work for this. That way the stop will be a little kinder to the parts being hit together when the motor works.
Google search will show you where supplies of Kevlar and thin epoxy resin & hardener can be obtained.
www.uscomposites.com/index.html is where I buy my frp supplies. since Kevlar is expensive and the minimum order is a yard, I suggest the 6 inch wide material @$5.25 a yard.