Hi Poxxxy! Thank you for your reply and Email. You've asked some interesting questions so I will try to answer them here. To build the car, there are 4 options.
First option is to source a 126 that's beyond economical repair (I.e, Major rot but running engine), Cut away the body and leave only the floor pan and arches as a rolling chassis. Replace and reinforce the floor pan where necessary (replacement floor pans are available on ebay for around £180 and steel arches are available from specialists on the net at very good prices). retain all running gear, wiring loom, steering geometry etc to transplant into the new shell. Add an OEM fiat 500 lighting pack and a trim of your choice.
Mate the floor pan to the underside of the new shell (fits like a glove) and fit the vehicle out. Roll over hoops would add extra strength. This will then retain the registration of the donor car. Registration with DVLA would only be to inform them the vehicle is a rebodied fiat 126 and is called a Barchetta 595. DVLA will update the log book and job done.
Second option is to have a bespoke chassis made out of box or tubular aluminium or steel. (MK engineering in yorkshire (Martin Keenan is the man to talk to as I have been liaising with him regarding this and the possibility of other donor cars like the smart fortwo-he is already familiar with the body but only a copy version without the lower tub).
Transpant the entire running gear to the new chassis and body (if you have the logbook from the donor you plan to transplant from, after an IVA test, it would be issued an age related plate. Not a Q Plate.
Option three is to build one with a new chassis and all new parts (a reconditioned engine will be allowed) and this will be registered as a new vehicle and a current year registration. A donor wiring loom can be used along with the original clocks as long as they are zeroed.
The last option is as option 3 but with any engine that will fit and what ever running gear that would suit your needs. Registration is in the same way - IVA etc.
Cars to consider as donors are:
Smart fortwo as the layout of the floorpan lends to this and there is only 27mm of difference in the wheelbase. This can easily be rectified by altering the two front wishbones to bring the front wheels in by 27mm.
A fiat seicento with the engine moved to the back and all other running gear etc.
I am also toying with building an electric version on a new chassis using a G Wiz as the donor. It will also be fitted with a 50cc range extender (engine mated to a 48v DC charging unit and controller).
When the batteries run low, the engine kicks in and give 3.1kw of direct to motor power and will charge the batteries when stationary. Great if you cant plug it in anywhere or want it to charge itself outside the pub or supermarket. This works better than mains charging because DC to DC charging at high voltage is far better than trickling of the mains. That's ok when your at home but not when your out and about. The range extender also add 40%+ extra life to the batteries as the engine will kick in and out with little top ups as you drive preventing low battery discharge.
A picture of a cut down 126 is at the bottom of my web page
www.595s.orgI hope this information helps.

Mark....