I had the same problem. After a discussion with JIM (Jira Importer) developer, I used YouTrack Rest API and Python script to make JSON files. Then I used JIM JSON import.
With this solution you can import almost all fields from YT - the standard one and files with description, links between issues and projects and so on...
I don't know if I can push it to GitHub, I have to ask my boss - I did it during my work hours.... But of course you can ask me if you want.