Mountfield M1 Super Running on unleaded?

I have a Mountfield M1 Super, I don't know how old it is, it belonged to an old gent on the next allotment to me, he bought it secondhand about 1976 it appears to be running well on unleaded petrol, I was wondering if there is an additive I can put in the tank or will it run as it is. The model number is 92902. Type 1789. 01 and the code is 78082825 any advise would be welcome