Visual Studio 2017 Community, .NET 4.6, C#
dotPeek: https://www.jetbrains.com/decompiler/

I copied libtetradec.dll, SDRSharp.Common.dll, SDRSharp.Radio.dll & SDRSharp.Tetra.dll (old hotfix #6 version) to a separate folder and opened SDRSharp.Tetra.dll in dotPeek and exported it to project.
tetra-dotpeek.jpg

... and opened SDRSharp.Tetra.sln in Visual Studio 2017 Community edition.
tetra-vs2017.jpg

I made several fixes to C# code in many .cs files. E.g. added missing breaks (switch-case clause), added some castings, changed BeginInvoke((Delegate) ... to BeginInvoke(new Action(() ... etc.
After rebuild all I tested it with SDR# and seemed to work.

Reverse engineering task is simpler (less to fix in code, only one (int) casting, some BeginInvoke fixes and adding missing breaks) when first decompile TETRA plug-in v1.0.14 and take rm3014.cs in a separate folder. After that decompile hotifx #6 version and replace rm3014.cs with v1.0.14 rm3014.cs.