This my old, very buggy, incomplete and slow NES emulator. But hey, it works on a Megadrive! It was written last year in March by using Stef's devkit, in C. But unluckily I've lost the source code to it.
Only mapper 0 ROMS are supported, and to change the NES ROM you simply have to replace it at the 512th Kilobyte of the Megadrive ROM of the emulator.