I'm currently building an Ultima GTR which uses a modified Chevrolet engine and a Porsche gearbox, both used components and the car, following the IVA will be issued with a new registration number, or a personalised reg..with the appropriate restrictions. All other components are new. There is all the info needed from the DVLA website and more grounded advice from the manufacturers.
If a new monocoque bodyshell or chassis from a specialist kit manufacturer (or an altered chassis or bodyshell
from an existing vehicle) is used with two original major components from the donor vehicle, a replacement registration number will be issued based on the age of the donor vehicle. The vehicle must have IVA, SVA or MSVA. The date of manufacture for the vehicle will be taken from the IVA, SVA or MSVA certificate.
Good luck with the build I will follow with interest.