Should be good with the bill of sale. I'm not familiar with the Illinois titles, but is there a spot for the seller to fill out (print & sign)? If so, was it filled out?
Also, even though the tag office can do it for you, it may be helpful to have a LEO complete a VIN verification form. That way, the DMV knows the bike isn't stolen.
If all else fails, there is always the Vermont route. I picked up a CB750 from my brother, but he had never titled it in his name. To make matters worse, he had misplaced the title and bill of sale from the previous owner. He had picked up a few bikes from a seller while he was traveling for work and they had been sitting in his storage unit for a few years, during which he had moved 3-4 times.
I contacted the State of Vermont DMV and explained the situation (I live in Florida, BTW). I sent them a Vermont Application for Title, a Vermont VIN Verification Form, a copy of the Bill of Sale, and a check for the registration fee. They sent me a plate and transferable registration. I took the registration to the Florida DMV and got my tag and title. IIRC, the fee for Vermont was $75+/-. They ended up sending me a refund check because they initially overcharged me.