We can confirm that unfortunately, it is not possible to provide the exact same set of JQL functions that exist inside of ScriptRunner for Jira Server due to the fact that a lot of the API's required to create the same JQL functions do not exist inside of Jira Cloud.
This is described in the documentation page located here which shows what JQL functions exist inside of ScriptRunner for Jira Cloud.
The reason we are unable to provide these functions is due to the restricted functionality and API's which Atlassian provide inside JIRA Cloud, due to the fact that Atlassian only provides a Rest API in Jira Cloud and not the same Java API that the server version contains.
You can see more detailed information on the differences between ScriptRunner for JIRA Cloud and ScriptRunner for JIRA Server inside of the documentation page located here.
The Comparison With Jira Server table on the documentation page located here shows what functions exist in Jira Cloud and what ones do not as we have been unable to recreate them.