Good day,
How are you?
I've been working with php content management systems (like Joomla) since the 1990s. And I have a professional emphasis on information security for Fortune 500 banking companies, where I would do these types of clean-up and protection.
If this is a one-line malware /virus on Joomla, I can take care of it in an hour.
I can also let you know any security weakenesses once you do the upgrade. :-)
Sincerely,
John