I have been trying to find out the exact rules about catalyst requirement and it's a little vague. I know that most production cars from k reg onwards require a cat test but only if they are listed in the database. If you can't find the vehicle in the database you revert to non cat test.
If you have a kit car pre sva you certainly will not require a cat test, but there are another set of rules post sva and an mot tester will require the mac certificate which will have the necessary limits. I have seen conflicting information regarding requirement for cat test, some say 1992 and others have said 1995. I your donor is pre 92 no cat required (engine numbers and proof of age required). If it is pre 95 it may not require cat but i'm not sure of that, i'm just hoping thats the case.
The 02 sensor is a closed loop sensor and does not require cat for correct operation more the other way round. If you remove cat you still require 02 sensor in system or you will have excessively rich mixture.
A universal cat is available in the aftermarket and I think marlin supply the correct one for the job.
Brian