You could go for halfords and find a fan belt for a mini 1000 with an alternator. Thats what I did for mine. They have a little book showing which belt to use for which car.
There should be a number on the old belt stating its length
Onne van der S. MMOCno 60520 Moderator
2dr 1971 White DAF 55 (with hopefully a 1600cc engine soon)
2dr 1973 Bergina (DAF 44)
2dr Estate 1975 DAF 46 in red
2dr saloon 1972 DAF 44 in Mimosa
Although I bought a mini fan belt I used the original dynamo size one on mine. It fits fine and the alternator is midway through its adjustment. You may find the standard size fine.