The change from the existing mechanically driven hydraulic assist system to an electrically driven version is obviously quite a simple step, you just remove the existing pump and drive mechanism and replace with a suitably positioned and plumbed in electric pump. The rest of the system is entirely unchanged. Hey presto - less parasitic loss.
I believe that there are conversion kits available that work on the 'electronic power steering' system (i.e. all the work is done with an electric motor and no hydraulics involved) but these are aimed at cars with NO power steering at all, and simply splice into the steering column somewhere between the wheel and the rack.
There would seem little point (although I don't see it being impossible to do) in removing all the components of the hydraulic assistance system and then splicing the electric power steering kit into the steering column?
Link to overview of assistance systems:
http://www.caranddriver.com/features/electric-vs-hydraulic-steering-a-comprehensive-comparison-test-featureLink to after market 'electric power steering' systems (aimed at cars which currently have NO assistance system):
http://www.ezpowersteering.nl/language/23/2/Home.htmlForum article by someone having the ez system fitted to their Countach (ladyboy!):
http://www.ferrarichat.com/forum/lamborghinichat-com-sponsored-cats-exotics/351715-fitting-power-steering-my-countach-qv.htmlJust to add I haven't completely read through all those links and have winged my post a bit on stuff I vaguely remember so please feel free to correct if the link material outright contradicts my ramblings