Well, I had a look in my RTR spares box and found just the ticket for you

It a backplate for .21 / .25 / .28 RTR motors. It has a groove cut out of the face of the backplate for the pull start pin (on the end of the crank pin) to rotate through. Now, I am assuming the RTR and the Sport series are the same ?

The part numbers are

Rear cover: 28-1000
Rear cover O Ring: OR-0027
Screw set: S6-2606

I sell these as a kit, as the screws are a different length than the ones that hold the pull start on.

Please check with your Aussie GO supplier first and see if he has these parts.
If he doesn't stock them, and he doesn't mind me posting them over to you, then we can do it.
I don't want to step on anyones toes over supplying their customers.

Let me know the outcome, and hopefully we can sort something out for you.