Hi
Neat! you mentioned you guys are already on a rewrite path. That is the proper way forward and I recommend gathering as much information about the product and edge cases as possible. Document every business rule, every product decision.
Handwaving aside, audit the Foxscript source code, clone it to a virtual machine or container. Any issue you face, feel free to post an issue on Github as I would love a few edge cases on the wild.
As the Foxscript runtime relies on 64 bit offsets for the DBF/Memo, once a DBF (past the 2GB) is opened they can't be migrated back to old VFP9, so I recommend to test this carefully on an airgaped setup with tests DBF.