Caterhams would be a good donor, it's almost made for the job
Seriously though......
Our demonstrator is going to be Nissan powered, but only the N/A version, we're aiming for 200BHP.
Later Saab turbo units will fit on the Omega box without too much hassle.
Geert at AlphaX does a conversion kit to mount the Audi/VW 20v turbo unit to the ford box.
And don't forget the MX5 is plentyful now, bound to be a few in the brakers.