Tryin' to update my old Pro Tools 5.1.1

I have an old Pro Tools System; 24 Mix/Plus, (Vers 5.1.1). My single Mac G4 died, then, I removed the main hard drive with all the software inside, and installed it as a second hard drive in a dual G4 that runs both OSX and OS9.

Because I have a lot of plug-ins, I don’t want to switch to OSX and the new generation of Pro Tools like LE, because I won’t be able to use my plug-ins and the old hardware (audio interface), etc.

Besides, I have many Pro Tools sessions that I need to be able to keep working with.

What’s the newest PT version I can update to (OS9) and how I can get one?.

Am I doin’ well about all of this? I’m really confused.

Any help welcome.