big bad jon

hi all, im about to have the scoobyclinic turbo back 3" exhaust fitted and i was going to keep the one the take off for mot time. my question is will i have to replace the whole of the original exhaust come mot time or can i just take a section of the 3" out and put a cat back in, this is assuming of course that the original exhaust will marry up with the new 3" any comments from people who have had experience of this would be well recieved!