I have looked at this but have not done it yet – there are a couple of problems (that can be overcome) that you need to consider. First is finding a short dérailleur that will fit – you can hang them from the rear spindle, and plenty of people have done this, but the factory brazed on a bracket further up the fork for the ‘Speed’ models. Unless your bike is perfect the rear forks are not that valuable and this might be worth doing. The second is gearing, you will have to find a suitable high range gear set – the factory have supplied as high as 9 tooth cassettes on 17″ wheel bikes and you will (probably) have a 16″ wheel! This can be solved but costs. Then the hub will have to be fitted into the fork and that will require spreading.
Probably worth checking out Flickr and the like for pictures of Moultons that have been done.