Is this just a matter of changing a file extension, or are we talking about implementing a new specification?

Is this the relevant repository for rule taker logic?
https://gitlab.com/xalgorithms-alliance/rule-authoring-software/Xalgo-DevRM-v1

RE: "Is this just a matter of changing a file extension, or are we talking about implementing a new specification?"

I'd register the .dwd extension with ICANN, as we're introducing a new file format spec.

RE: "relevant repository for rule taker logic"

You'll find RuleTaker from here:
https://gitlab.com/xalgorithms-alliance/rule-networking-software/prod-impl/-/blob/master/README.md

Also, the URL you referred to was the very old v1 of RuleMaker. Here is the current RuleMaker:
https://gitlab.com/xalgorithms-alliance/rule-authoring-software/xalgo-devrm-v4/-/commits/dev