Convert a windows C# .NET project to Linux/Mac

We have a C# project that works under windows, but does not work under Mono. We would like someone to convert the project ideally so that it works on both Windows and Mac/Linux. The project is a basic GUI that connects to a server and downloads and uploads some items. The actual source code is a about 4,500 lines with a couple of other libraries, particularly ZIP and RSS. We would consider moving to a different language should the bidder make a good enough case for the conversion, however I expect Mono would be the best option.