Considering he's in Chicago proper...it's like finding a needle in a haystack.
And there are body (and chop) shops around there who do nothing but illegal work. (Repainting stolen cars, fixing hit & run damage...) Besides, everyone has a friend of a cousin who's willing to do the work "off the books".
I'm very sorry to hear about your loss. Despite what I've said above, I'd give it a go and try to track down the ***hole. Lord knows the CPD will be all over this.
Good luck.