lima 59s

Bachmann have a really good centre motor both bogies powered drive in their 66. Hattons are practically giving them away. http://www.ehattons.com/StockDetail.aspx?SID=17963 You can probably use the light fit to provide lights on the 59 too...
Also it should be possible to sell the Bachmann body and Lima chassis as spares to further reduce the cost.

With Hornby re-releasing the Lima model of the 59 the quickest and probably cheapest option is to either purchase a cheap Hornby example or call up a spares dealer to see if they have the parts in, but Hornby have changed the actual motor so you will need to buy a motorised chassis for it (making sure you order the right 'colour' (grey/black)).

The Bachmann 66 route would require some modelling skill if you are to make it accurate, as the 66 and 59 chassis are quite different 'in the flesh'

Another alternative that is quick and easy to fit with only a modicum of soldering is the fairly inexpensive ModelTorque Lima replacement motor.

Hornby's 59005 (Black underframe - with the wrong headlight clusters) is £42 in Hattons, Their model of 59102 (grey underframe) is £39 and both should be DCC ready. A couple of Hornby's 66s are hovering around £40 too.