Not to sure on the situation in Vic but in NSW the exhaust (as well as some other things) has to be current with the ADRs that were in force in the year that the engine was made. For example a 1971 TA22 with a 1989 4AGE in it would have to run all the regs for 1989, exhaust, fuel system etc right down to the unleaded fuel filler. This is of course not done by everyone, but you get that.