A few options:
(1) Take Amtrak; CHI-SPI has a scheduled service 7 p.m.–10:24 p.m. This is a $54 train ticket on today's service. United probably won't buy a ticket for you. You gotta get to Chicago Union Station by 7 p.m. There is one earlier train but you probably won't make the earlier 5:15 p.m. departure.
(2) Insist to be put on the standby list for the 9:10 p.m. ORD-SPI — as a displaced full-fare pax you should have a chance of making standby even on a sold-out 50-seat aircraft (it's a CRJ-200 tonight). An airport agent can do this (anyone at the customer service counter or at the departure gate for the flight). This is scheduled to get you in at 10:04 p.m., arguably worse than the train.
(3) Rent a car and drive, a little over 3 hours. United probably won't pay for this.
(4) If you picked (1) or (3), get a refund for the portion of the trip that you didn't take (takes UA a while to calculate this, should be due something).