The easiest way is to ask another more experienced jet modeller in your area to have a look at the starter. Perhaps someone at your club?
The O ring is located in a groove at the turbine end of the starter. When the starter spins, that end of the starter is thrown out by centrifugal force, to contact the nut on the front of the turbine. The starter rubber ring is the contact point on the turbine & spins up the turbine to initiate the start sequence (also connects to cool down the turbine when you shut down the turbine) .
This video shows a typical starter. The video is about a Wren turbine, but the Jetcat starter works the same way. Look at the first couple of minutes & you will see the relevant working parts.
If the rubber O ring looks cracked, or damaged, replace it. You may be able to get one the same size from a hydraulic shop.
Check that the Bendix (called clutch in the video) is moving backwards & forwards freely.
Usually the noise of a starter requiring attention, is a screeching sound.
Do a search on this & other forums for servicing turbine starters & replacing O rings.
Here is the link mentioned earlier.
https://www.google.co.uk/url?sa=t&rc...r3a_6czKVhwzUg