Hi, another alternative is a Firefly or a Dragonfly. See:
http://riomobility.com/en/ I have a dragonfly which is cheaper than any alternative I have seen and works pretty well. The biggest disadvantage with this kind of cycle is that there is not much weight on the front wheel which means no traction going up hills. The dragonfly will only handle a slight upslope so don't expect too much. PS If you are trying to get fit why do you want a motorised attachment?