good morning guys,

im just trying to get me head around this q plate registration.

if a kit is built using a single donor and therefore is given a age related plate what happens if later down its life you change the engine and gearbox form a completely different car.

does the car still keep its original age related reg once the new details are passed to the dvla?
also does the car have to go through the whole iva test again?

thanks for the help given.

You only need to IVA if modding the chassis. Changing engines may change the tax band but will not require IVA or re registraion

Athough, Im not sure what happens if the gearbox is a stressed member, or even if that is allowed through IVA!

If you change the engine you have to inform DVLA and that could result in a change of taxation class. No change in registration. There's ne requirement to inform of a change of gearbox (no details of the box arekept by DVLA) and it has no effect.
The only reaso for a re-IVA would be a major to the chassis, but even then it'd be down to you to inform them and they have no record of what your chassis design was at either the original IVA or registration, other than the chassis number.