Are router mounting holes standard?
They are not standard, but there seem to be a few common patterns. 

Rockler's website has a guide for choosing a mounting plate for your router. These are the router models that they support:


  
  Group A routers: Makita 1100; Milwaukee 5615-5624; DeWalt 616-618; Bosch 1617-1618; Ridgid 2930 Combo Kit; Hitachi M12VC (fixed and kit); Porter-Cable 690, 890, 7529, 97529 and 8529. Note: Current models of the Porter-Cable 892 require at least two screws  to be 1/2" long versus 5/8".
  Group B routers: DeWalt DW621 and DW621K; Craftsman Standard; Freud FT 1700E and 1702VCEK
  Group C routers: Porter-Cable 7518, 7519, 7538 and 7539; Milwaukee 5625-20; Triton TRA001 and MOF001
  Group D routers: Bosch 1619-1619EV; Freud FT2000; DeWalt 625
  


They also offer blanks that you can drill to fit your router if it's not in the above lists. 

On the "Tech Spec" tab of that page, they have PDFs that show the pattern for each of the four above groups, so if you measure the spacing of the holes in your table, you may be able to find one that fits.