So if we think about what sort of ribozymes could actually exist and function in a RNA world it seems me that the very first thing one would need to do is study how well ribozymes function without cytosine. One paper that attempts to do this conceptually can be found on the link below:
If we take the short chemical half life of any cytosine generated in the warm oceans of the Archean, and the lack of cytosine in meteors, then one must acknowledge that ribozymes taken from nature that rely on 4 RNA nucleobases, do not represent chemistries that would be possible in a hypothetical RNA world. So chemists would have to first engineer alternative molecules that are capable of analogous processes accomplished by observable ribozymes while just using three bases first. Then it would necessary to demonstrate that those compounds could last long enough to reproduce, and be capable of guided evolution.
Current experiments that attempt to use guided evolution to engineer riboorganisms by using RNA enzymes that are taken from living cells, are using templates that are not demonstrably able to exist without cells that function in much the same way as modern cells do, and are thus examining how to use current biology to engineer alternative biologies, as opposed to studying how the raw materials available in nature could have been the basis for an RNA world on the early earth.