logbook title, can i change the name of motor?

[digler]

Hi all,
at the moment i'm building a classic mini clubman estate, but with a twist!
I have put the complete floor and drivetrain from a mazda mx5!
All the running gear is standard as are brakes, suspention and steering.
Ive applyed for a new logbook for the mini, and i have the one for the mazda.
I'm not sure which logbook to use?
Do i use the mini and get the old plate? 1979
Use the mx5 plate, ( would it need an iva test, with the point system? If ive only changed the body?
And if i used the mx5 logbook could i rename the car on it? So that its not a mazda or mx5?
Thanks if you can help, i'll put a few pictures up soon.