Another option as a replacement is some rubber tubing, which allows for flexibility of the drivetrain that the plastic coupling will not. This is really only something worth worrying about when mounting the motor or shaft bushing in the car, but I like having the wiggle-room when reinstalling the motor.
Here is a shot of a Monogram Porsche 550 that Harry had tuned with the tubing, along with the stock spring on the left:

I got my supply of the stuff from him, so can't speak to its specifics or where to find it. I just refer to it as
Harry's Magic Tubing because it quiets down and improves the response of every car I've put it in.