I would have to say Yes.
Alot of us still drive cars without both of those gizmos
the seat belt will still work it just maybe wont pull your waistline down by 6inches and so you dont have a pillow to scream Fukinell into when it all stops
i wouldnt bother, mainly because of the cost
also, if you crash hard enough that an airbag would make a lifesaving difference, the older car will have probably deformed enough that you have other, more important things to worry about
Basically, unless you fancy buying new replacements. In reality the new replacements were probably manufactured a decade or more ago anyway, so I think it's best to assume that the pyrotechnics might not work, and drive accordingly.