Compulsory track time insurance for "recreational closed-circuit motorcycling" is mandatory. This should cover repatriation and compensate you in the event of an accident. This is not included in the standard health insurance.

So what you really need is some proof that you are insured for this activity. A certificate or copy of policy conditions is OK. Make sure you bring this on your trip! An air ambulance can cost up to €40.000...
Also please ensure you bring your European Health Insurance Card (EHIC)! Without both these documents, you will not be allowed on the track.