I have looked back at my versions and you are right, the current rules don't detail solo play. This was edited out of the original version of the rules during play testing in order to keep the size down, and was never replaced. Apologies.
I have taken the draft notes for them and produced a separate set of solo rules with 3 variants. This was uploaded this evening and should be available to download very soon. It would be nice to expand this further so please feel free to let me have any suggestions for further variants.