First, you're not going with Modbus+ unless you go the second (or third) hand hardware route.
Your hands are going to be a little tied unless your PLC has Ethernet in some form. You could use a TCSEGDB23F24FA (doesn't that part number just roll off the tongue) which is a Modbus+ Proxy module for the M340 and M580. It's designed to allow M340's (and 580's) connect to a MB+ network, but it will route data the other way. It does stupid things with the unit ID in the encapsulated Ethernet packets, so it actually wouldn't work with a Schneider ATV71 VFD (also obsolete) because - Schneider. I haven't tried it with a AVT630/930 etc. yet. Message me if you'd like details.
In theory it would allow you to replace the ATV66 with a different drive that can speak Modbus TCP. You'll have to change the register addresses and scaling in the original PLC to match what the new drive needs. Again, I tried it with an ATV71 and it didn't work due to stupid programming decisions in the Proxy module firmware. That was years ago - maybe they fixed that. Ha! We know they didn't.
You could perhaps use serial Modbus communications from the old PLC, assuming you've got a Modbus port to spare on the PLC.
Now, if you're desperate and need a hack and slash solution that you never heard me say:
Wire digital and analog IO from one of the drives that still works to a new drive and modify the PLC program to control and monitor the IO in the existing drive to control the new drive. Start/Stop, speed command, running feedback - what more does a guy need?
Come on - we've all seen worse!