What reg plate for a Bike engined car?

[wollyd]

Hi all,
I'm looking to build a kit car in the not too distant future. Looking at an MK Indy with hopefully a fireblade engine. My question is what number plate will it bear? I thought to be age related you needed to use the engine from the donor vehicle along with other specified parts. However I've just seen an MK with a bike engine on Ebay and it has an 'F' plate. Confused, help!!