IVA/Registration number confusion

[hyperbodger]

Apologies - this topic has probably been done to death! Just starting building and have spent weeks on the internet trying to understand the basics of getting either an age-related or current registration. I don't understand exactly how a donor car's number is given when the chassis, bodywork etc probably look nothing like the original. I'd like to (try) to get a current registration (budget permitting) but can't see how to do that either because the whole diff/rear axle etc will probably need to be recon as I don't think they're available new - and most gearboxes seem to use reconditioned cases. So - any advice/help/experience will be really appreciated!
Thanks