Last year I went searching for a better language for cross platform development. In the end I found that Python was an excellent choice. It handles all of the issues you normally have with GUI and files system differences between OS's. Not that python is the answer but it is a possibility. If you're not up to attempting a port then would you consider open sourcing the code to another team? I realize you have a lot of code and time invested but a motivated team could probably pull it off.