Even at 21 you may struggle to find a insurer wishing to offer a classic policy for you, classic insurance is more about you than the car, eg it's usually for those aged 25/30 or over, second car , limited mileage, no or few mods etc.
You may be better looking for a limited mileage policy.
Also, with regards to trackday cover, you may find the excess on a trackday policy is more than you actually paid for the car. If the car is worth under £5000 the general rule of thumb is, you bend it, you mend it.
http://www.skyinsurance.co.uk/car-club-insurance.htmlOllie
Sky Insurance