Your best bet would be remapping, like TLMB said.
When I started out inserting custom sprites into my hack for the first time, many of the tilemaps for the sprites disagreed with the tilemaps of sprites I wanted to use in certain levels, so to make sure they could stay with the custom sprites, I simply used tiles I wouldn't use for the level when I remapped the custom sprites.
To remap, you just have to change one or more of the values in the asm file (but you need the $ signs there), they should correspond to the last two digits on the pages with sprites (e.g. if you saw C8 in the tilemap, it would correspond to the Fuzzy enemy, which is tile 3
C8 in the tilemap).