I did manage to get SharpDevelop to compile and run SB-Prime. 
1] Need to change csproj files as described before for ToolsVersion 4.0 and TargetFrameworkVersion 4.5.2
2] Fix a few minor syntax differences for earlier C# version - I have actually made these changes and checked them into git since they are fine in recent C# version.
3] Target CPU prefer 32 bit (Properties-Compiling)
4] Add a reference to System.Runtime 4.0.0.0 to SB-Prime project
5] Add system environment variable (your location may be different) TargetFrameworkSDKToolsDirectory = C:\Program Files (x86)\Microsoft SDKs\Windows\v10.0A\bin\NETFX 4.8 Tools
6] Set SB-Prime as startup project
Compile as debug or release both work for me (Build->Set configuration).
By starting from 'scratch' I mean creating your own project from the beginning with a window, maybe RTB and events for it. You will learn more by creating from the start rather than copying code, although this can be helpful too. It will be hard to modify a big code without understanding of how it works and the language syntax, which you get by writing your own projects 'from scratch'.
1] Need to change csproj files as described before for ToolsVersion 4.0 and TargetFrameworkVersion 4.5.2
2] Fix a few minor syntax differences for earlier C# version - I have actually made these changes and checked them into git since they are fine in recent C# version.
3] Target CPU prefer 32 bit (Properties-Compiling)
4] Add a reference to System.Runtime 4.0.0.0 to SB-Prime project
5] Add system environment variable (your location may be different) TargetFrameworkSDKToolsDirectory = C:\Program Files (x86)\Microsoft SDKs\Windows\v10.0A\bin\NETFX 4.8 Tools
6] Set SB-Prime as startup project
Compile as debug or release both work for me (Build->Set configuration).
By starting from 'scratch' I mean creating your own project from the beginning with a window, maybe RTB and events for it. You will learn more by creating from the start rather than copying code, although this can be helpful too. It will be hard to modify a big code without understanding of how it works and the language syntax, which you get by writing your own projects 'from scratch'.