That should be a simple upgrade you have most of the hard work done in the form of mountings etc, just a few adaptor plates for different size frames with motors will sort it.
I'm presuming you don't want to CNC the Knee.?

You just need to decide if want servos or steppers.? Quite a lot more work in setting up along with big learning curve involved with servos but they do give higher feeds, more power along with smoother operation and accuracy.
The downside is they also cost more money and require a higher level of the controller to get full use out of them. They can be much more temperamental than steppers and very unforgiving on poor wiring quality or practices.

Steppers, on the other hand, are much simpler to set up with hardly any learning curve involved and quite a bit cheaper. They can be run on a lower level of controller.
The downside is they are limited to roughly 1200 to 1500Rpm so feeds are lower. Resolution is lower compared to servo's but still more than enough for milling at this level.

Choosing the correct drives and size is critical with large steppers because you don't have lots of spare power as you do with servos. So if you take this route then look for 220V Mains powered Drives rather than lower AC/DC drives running say 80Vac or 100Vdc.

I've used these many times and they are a great set up with lots of power and provide reasonable torque even at high feeds. More than enough for your machine.

https://www.aliexpress.com/item/3279...3cac6f37OxW5VA

Hope this helps, got any questions just ask.