Okay so if you have a V5C from a pre 1992 car and you intend to use the A series engine as shown on the V5C in the buggy then you will not need a cat. If you use another engine which is not on the V5C then separately you will have to prove the age of the engine to the both VOSA at the time of the IVA so they know what limits to set & then to DVLA for registration.
My own situation is that when I did the SVA & registration in 2009 I used the engine that was listed on the V5C, told then that I had used the engine, gearbox (no choice on A series) along with steering column/rack along with suspension parts & brakes, this was enough to convince the lady at DVLA that it could go on an age related plate. Since then however I have removed the 1987 A series & fitted a K series from a 2000 MGF I have a cat fitted & yes at MOT time it gets tested as a 2000 car, although the garage I take all my cars to is clued up & does things correctly. Currently on the V5C it is still registered as a 1275cc, I did try last year to advise them that I had fitted a 1796cc engine, however failed, I gave them 8 weeks to do a simple task before I needed to tax it get back on the road. But by the time I needed to tax it they still hadn't done it, so I rang them & asked what to do, I was advised to tax it & they would then just send me a invoice for the difference. However they rejected the application as I had taxed it FFS, they also needed proof of the engine by either AA, RAC or some official body since then I haven't got round to doing it.