The good news is that the dust-cap itself isn't that big of a deal as far as the operation of the speaker goes. If you dent it in too far, and then play it loud, it could bottom out. (which would be bad) But it's otherwise just there to keep crap out of the voice coil gap, maybe force air around for different cooling schemes, and look pretty. Some claim to help "stiffen" the cone, but all the ones I've seen make that claim are inverted and thus already "dented" in.
You should be able to get a new dust cap and replace the dented one without drama. A generic cap may not look as cool as whatever it came with, but it should "function" as well as the original and defiantly look better than the dented one you have now.