Integrate custom from AX 2012 C# Project to D365 for TMS engines

In AX 2012 R3 cu12, we developed our own internal TMS engine to call 3rd party web services.  In AX 2012 version, this required us to customize from the AOT >Visual Studio Projects > C Sharp Projects > Microsoft.Dynamics.AX.TMS

Within VS, we modified (add 1 line of code) Utility > XDocExtensions.cs

... Now in D365 I have found the C# class under AOT > References > Microsoft.Dynamics.AX.TMS.Base which is where this has been moved.  How do we go about making changes to integrate our code?  LCS just left this out when we ran the upgrade analysis.