My company has a large C# code base. Well over half of this code is our core engine for creating, reading, modifying, calculating
I think you will find that the Mainsoft, Enterprise Edition tool allows you to run most/maybe all your .NET code under the Java JVM... Seems to be focused more on ASP.NET but will allow C#. It has been available for some time, pity they don't publicize it better!
Warning blurb follows....
Mainsoft® is Java-.NET interoperability software that enables IT organizations to move to Java-enabled platforms such as Linux while preserving existing investments in .NET code and skills. The software integrates seamlessly into the Visual Studio® development environment, enabling C# and Visual Basic developers to rapidly develop and maintain server and Web applications that run on Windows, Java EE platforms or both, thereby reducing application development and maintenance costs, time-to-production and total cost of ownership.