Usually, mono packaging includes a referenceassemblies-pcl package, which is currently missing in Solus. I know that, since they provide no stable release, it's expected to be missing, but as it's binary data, it should be included (it's not compilable per se).

Upstream can be found here: https://github.com/mono/reference-assemblies and it seems to be distributable under the same terms as Mono.