After some difficulty obtaining reliable and correctly-made supplies of Emco Unimat belts we have decided to offer instead the special Swiss-made drive belt we sell for light industrial applications. It's widely used on all sorts of drives and we now sell lots for watch and clockmaker's lathes - and of course various kinds of Unimat. It's long-lasting and very reliable.
The diameter required is 5 mm and if you order 1.1 metres you will have a little left to practice the joining technique. 1.1 metres is enough to make 4 or 5  belts and it's joined by melting the ends together - simple instructions provided.
If you need more than 1.1 metres you can order the same belt here: http://store.lathes.co.uk/belts/solid-round-plastic-belting.
As the belt length on the Emco 3 seems to have changed over the years, if you can measure the lengths needed, we can join them for you. Phone: 01298-871633 (from overseas ++44-1298-871633 to discuss)