Decatting is strictly for the birds. It will gain you very little power unless you remap the ECU to run very rich (it'll almost certainly fail emissions if you do).
So, run a cat. You'll need something like the SPI cat. The Rover KV6 cat is about the right size and shape and shockingly cheap on eBay. You then weld a threaded boss (all the exhaust places sell them) after the CAT and fit the post CAT lambda onto this.
A silly bodge which seems to work is to wrap the second lambda in aluminium foil.
BUT an MPI car without a cat (or, I guess, with a lambda wrapped in aluminium foil) will fail the MOT unless you're having sex with the examiner. And you won't be able to have sex with the VOSA examiner at the spot check point 'cos the police will be there ready to arrest you for whatever......
Be aware that the late MPI ECU is fussy, the early MPI ECU (the micro ECU) fussy, very difficult to work with, and unreliable.