Hi does anyone know if the oil bath filter can be changed or removed so that a convention air cleaner can be used instead by attaching straight to the carby for example?
Why? The 504 oil bath filter is superior to any you could replace it with. There is an old article in Torque by Bruce Taylor explaining the superiority of the system. And explaining why the foam rubber filters installed on some later models were suitable only for lawn mowers.
Oil bath more efficient and French manufacturers often doubled up with them on export models - my 4CV has initial oil bath filter then a cartridge. Our dusty roads in the 50’s got a bad rap
They are also cheap - clean the steel element, top up oil bath with oil ( doesn’t need to be good or even new oil ) . Fit and forget until next time