The honest thing to do is send the cossie V5 away saying it's scrap as that's what it is now.
buy another cosworth shell with v5 from a breaker or eBay etc and rebuild it with your parts if you want to rebuild it.
by the time you strip both cars,get the donor car painted the colour the cosworth was,modify the chassis then rebuild it you're giving yourself more hassle and expense than you would be replacing the car with another assuming you got a good payout and the car back which would prob break for about 8-10k depending on spec.
dont agree with re-shelling unless it's a brand new unused shell,anything else is ringing as you are swspping identities over which is illegal.