I don't see this as a bad idea; I think you're obliged by law now to disclose engine no. changes to the DVLA anyway - I had to notify the change of capacity and engine no. when I did my Capri.
The way I see it, if you've got nothing to hide and swap an engine it will only cost you 10 minutes to fill out and post the V5 to the DVLA, and 30 seconds at MOT time when the dude checks the numbers.
The only times I imagine you'd want to hide the engine no. is if it's a) nicked or b) something hot you don't want the insurance company to know about. a) You should know better or you're pikey scum
b) Again you should know better, as it will void your insurance and screw things for all concerned if you have an accident.
I don't usually like tightening of regs, but for once this seems reasonably thought out. As long as it doesn't start restricting mods etc it should be fine.
Of course, I'm sure those who want to avoid the no. check might just boycott the MOT all thogether?
