In ProSolidMassPropertyGet, there is an option to get the mass properties for a solid relative to a desired CSYS. The function takes a CSYS by desired name, however it only works relative to that solid (there is no support for gathering the mass properties of a component in an assembly). In Creo 2, the Analysis-> Model Report -> Mass Properties tool performs the calculation based on a selection to a CSYS.
This difference is important because Toolkit lacks the capability to calculate all massprops of a component relative to a desired CSYS that could potentially live inside of some component path.
Please either change the function to use a ProSelection (or at an Asmcomppath and a Modelitem/ProCSYS individually), or create a new function to be able to gather the mass properties of a component in an assembly. Another alternative would be to add "PRO_MASS_FILE" to ProOutputFileWrite, to generate the same report as is generated using Mass Properties -> Display Info, where one argument1 is the ProSelection to the desired CSYS.