Generally, if any modification is done by the owners of the vehicle, then the warranty of that vehicle would lapse. So if you want to do any modification to your bike, it will be at your own risk. If any further problems or issues occur in your bike due to modification then it won't be covered under warranty. Still, it is suggested to discuss with the nearest authorized Royal Enfield bike dealership. A dealership executive will guide you and give you proper information if you can install Kick Start externally or not.