Don't throw out that motor! Don't worry. You can use it.
Just get a new grip. That's what I did with my PDW. Check it out:
This one has the G&P M170 in it, long type.
edit: I should add that in many many cases, on the classifieds, in people's spare parts bins, etc, you can either get grips for free or as little as 3 bucks. Don't need to spend much.