Kitty Litter?
Actually, acids are pretty good a breaking down the proteins that smell. Vinegar is safe. Vinegar is cheap.
To be effective, you'd have to take it apart and ensure you've removed all left-over mouse parts and any foreign material (grass, leaves, dirt, mud, etc.) that may have absorbed the smell. Then wash down with vinegar. Vinegar is cheap enough you could even soak the heater core and duck work in it.
There are bags of crystals you can buy that will absorb odors (but not remove their source, of course). When saturated, you simply bake the crystals to "recharge" them (heat releases the previously absorbed odor). Do it in the sun, not your indoor oven (unless you want your oven & house smelling like mouse).