Difference between AR2 and AR3

The primary difference is the dual shaft motors, encoders, teensy 3.5 control board and updated wiring and print files. You can definitely use the same enclosure.

So the bulk of the cost of upgrading is just the motors it sounds like https://www.omc-stepperonline.com/only-motors-of-ar3-open-source-robot-package-kit.html I am about 85% done w/ my AR2 but it got stalled when my dad got sick...I think I'm going to just go for upgrading it to the AR3 since I can re-use so many of the parts. At first I thought I needed new aluminum kit, hardware kit as well as stepper drivers.

I find these stepper motors are similar you yours:

https://www.oyostepper.com/goods-863-Only-Motors-of-AR3-Open-Source-Robot-Package-Kit.html

https://www.oyostepper.com/goods-862-AR3-Open-Source-Robot-Package-Kit-Stepper-Motor-Driver-Power-Supply-and-Bracket.html