Fixing Old Legacy Code

I inherited a very old ColdFusion Code which ha Application.cfm file. Now i need to handle the 404 error.  I cannot modify the IIS directly and nor i have the Access to the Administrator, What is the Best possible way to do it.

I cannot convert their Application.cfm to Application.cfc as it is too big and segregated across all the pages

So this is something where i am stuck can anyone have any idea how can i write some piece of custom code to handle 404 with coldfusion