Hi

I need to get my v5 updated with the new i engine i've put it in but where do i find the engine number ?

You put an engine in,and didnt take the number down first? Well if its not been filed off with an angle grinder,you'll find it on the side of the block just above where the top of the gearbox bolts to it.

What happens if you don't have the engine number? Does the car get Q plated, or can you simply not re-register it?

theres no number on the side or i might be blind, is it really obvious ?