Legal 2 with an illegal basement apartment - Chicago

[Kavan Kucko]

I am looking at a property that is a legal 2-unit apartment with an illegal basement in Chicago. I ran the numbers and it looks like a good deal, even without including the income from the basement (~$500/mo positive cash flow when trying to overestimate costs). So I don't need the income from the basement, and I'm not really interested in renting an illegal basement, but I'd much prefer not to evict the basement tenants. The two guys living down there have been living there a long time; they might have even previously owned the house. 

I am not concerned about the city finding out and having to pay a fine, and then having to evict under those circumstances since there's cash flow without the basement rent. My concern is that of larger liability - think fire or something like that. I'm not sure exactly what makes the apartment illegal, it has a legal exit (and is small enough that I think it only needs one), the windows might be too small and the ceiling might be too low.  

I think I may purchase the building, rent to the two guys in the basement as long as they'd like to stay, and if/when they leave, either look into making the unit legal, or quit renting it. But I worry even that may be exposing me to too much risk.

Just wondering what people's thoughts are on the liability vs kicking out long term tenants.

Thanks