You could. Maybe not by the commonly available ROM patching tools. But you could write a program that takes as input the NES ROM and outputs the C64 ROM, such

You could. Maybe not by the commonly available ROM patching tools. But you could write a program that takes as input the NES ROM and outputs the C64 ROM, such that that program contains nothing copyrighted by Nintendo. It would convert the level layouts and tile/sprite artwork and sound effects and music and so on (maybe even the 6502 assembly routines for jumping and physics and such), from the NES ROM to whatever the C64 port will use natively.