i have a vw van 1978 camper type 2 they are telling me to get a catalytic converter?? on 1990 Volkswagen Vanagon

At the smog check this year the smog place is not passing me because of not having a catalytic converter on my 78 VW camper van. I have checked for catalytic converters on line but am extreamly confused as to which one or how and if it is only the converter or do i need more parts total?
Is there help to convert the van to catalytic? A program? what total parts do i really need? this is the CA ones only allowed as well.

Asked by for the 1990 Volkswagen Vanagon