Have a look at the website below for NF Auto Developments. They make replica P4 Ferraris and are about 10 mins down the road from me.
As with a lot of kit builders they use the PRV V6 engines and UN1 gearboxes as one of their options (
http://www.nfauto.co.uk/options.htm ) but do a fair few with genuine Ferrari V12s. I asked wasn't that a bit expensive? Apparently not - they tend to pull them from old Ferrari 400s that are beyond hope and can be bought for a few grand.
I went to speak to them a few years ago about possibly getting the GTA re-sprayed, and they're really friendly guys. As with most enthusiasts they were pretty impressed with the GTAs design and longevity. I just appreciate the fact that they do proper engineering - it amazes me how many garages don't even carry the simplest things like tap and die sets.
Check out the for sales - one PRV V6 with UN1 plus a V12 Ferrari with the Porsche G50 gearbox.
http://www.nfauto.co.uk/forsale.htm