The V5 stays with the frame number. You can swap engines on the document
A frame with a V5 usually fetches upwards of £600 or so............
eBay usually shows a few every now and then. Personally, whilst I see why some people break perfectly good bikes for spares, watch out for ones being broken due to a tpft accident where the owner can't afford to repair. I'd only buy a frame from a "known" source.
It should be possible to HPI a frame against the chassis number and V5 document though.