Modifying tilesets?

[s]So here’s the thing: Mshock is really falling behind, and more and more my vehicles are looking like unrecognizable piles of ascii. ChestHole corrects a lot of this, but I just can’t handle the colors they chose, the way doors look, or the bobbleheaded zombies.

What I’d like to do is take Mshock but make it display roller drums as the same tile used for armored wheels, rather than a letter B, maybe some other similar changes. How do I go about this?[/s]

Or just use retrodays, growing oddly fond of it.