Map Fn + Esc key to Esc key
Fn is a function of the keyboard that modifies the key-codes being sent by the keyboard itself. Fn itself does not transmit any key-codes to the computer.

One option would be to try mapping the actual key (that is turned into Esc when Fn is pressed) to be the Esc key. I don't know of anything built into OS X that will do this across the board, however, you could try something like https://pqrs.org/macosx/keyremap4macbook/

To be honest, good keyboards are so cheap these days you should probably just buy yourself a nice new keyboard that has a layout which suites your needs better.