From the Straight Dope:
Constructed and furnished for $115,000, big money at the time, the Illinois rested on a foundation of piles and timber, on top of which sat a faux hull made of brick covered with concrete to give the appearance of steel. The superstructure, including guns, turrets, and armor, consisted of wood framing and metal coated with cement.
The rest, by and large, was real, including the decking, masts and smokestacks plus, evidently, many of the smaller armaments, such as Gatling guns and breech-loading cannon. There were engines, boilers, a steam launch, boats, anchors and chains, searchlights, winches, cabins and mess rooms, a bridge and charthouse, and everything else you’d expect to find aboard a mighty warship — even a brace of cutlasses and revolvers. A trench had been dug in the lake bottom so that the Illinois could launch torpedoes at any enemy craft that strayed into its crosshairs. (What, if anything, they were actually launched at I haven’t been able to discover.) The Navy detailed a crew to wear old-time uniforms and explain the the ship’s intricacies and exhibits to the gawkers who crowded the decks.
It was a grand spectacle — maybe a little too grand. People began to get big ideas. After the fair closed, the Navy donated the replica and its furnishings to the state of Illinois. Soon a scheme was afoot to move the Illinois to a new berth at a pier off Van Buren Street, where it would serve as the headquarters for the newly-formed Illinois naval militia.
USS Illinois* was built to roughly the same specifications as USS Indiana (BB-1); Cecil argues (correctly, I think) that the ship was part of the general PR project that the Navy undertook in the wake of Mahan’s The Influence of Seapower Upon History.