I have released the source code for fMSX 3.7, including the Unix/X11 port. You can get it off the fMSX home page:

http://fms.komkon.org/fMSX/

Please, note that the following terms apply to the fMSX source code use:

• You can freely use fMSX sources for any non-profit project.
  
• You cannot use fMSX sources to obtain any kind of profit, either directly or indirectly.
  
• When using fMSX source code, you have to make a clear reference to me, Marat Fayzullin, as the author of the original code.
  
• Please, inform me if you are using fMSX source code. While not a requirement, it is nice to hear from you

Best regards,
Marat Fayzullin