http://www.ebay.co.uk/itm/MICROCAR-AIXA ... 337fb59026
The driven pulley will be on a taper so ask for the tapered adapter to make your life easy when mounting. You could ask for the belt as well just for setting up purposes(around 250mm distance I think) Belts are expensive!!
The rear pulley should be 19mm so a suitable shaft can be made or bought.
This is the petrol version so the weights on the driven one may have to be changed to diesel weights. These are available at Aixam dealers and only cost a few pounds.
It is quite bulky so would suit the larger bikes or if mounted sideways possibly shaft drive.
Good price though!!