If you are looking to run on a non-Windows platform and upgrading is not an option, you might be able to purchase a single user base PxPlus license to install and run independently of their Infor application. You can then have the application INVOKE that version of PxPlus to parse the JSON. You could place the JSON into a temporary file, invoke the newer version of PxPlus to parse the data and return the values.
Depending on what you need to do, you might be able to do additional processing in this new version however it would not be able to access any protected INFOR data or programs.
Basically, you would just be using this newer version of PxPlus much the same as if you wrote a program/interface in C, C++, Java or any other programming environment and invoked it.