As you know, the standard tire for the Riviera was 225-60SR16.
Using a 'specification sheet' you can cross-check the design rim width, diameter and max load of different sizes of tire to pick a tire of a different profile that has at least the same diameter and load of the standard 225-60SR16 97S tire for the Riv.
The link goes to a catalog for a line of tires from TBS International. They have tires made for them by various manufacturers. You may be able to find comparable 16 and 18 inch tires on one or another of the specification sheets in the catalog. See the LINK.
The point is you will see that there are trade offs in terms of size and load. I think the lower load on the low profile tires could be an expensive problem for you, in that the similar sized tire seems to be lower spec, but I'm not you - check it out for yourself by getting the spec sheet for the tires you wanted and check the actual sizes and load ranges.