This cut from the Warhammer Wiki:
The Empire War Wagon is an extremely heavy version of a chariot, a mobile armoured battle tower with a crew of six warriors, pulled by two sturdy warhorses. The high walls of the tower protect its crew from arrows and attacks from the ground.
War Wagons are a movable platform from which it either transports Empire soldiers through the battlefield or provides a vantage point for units of archers, crossbowmen, and handgunners to shoot the enemy. Its crew can also be armed with a variety of experimental weaponry from the arsenal of the Imperial Gunnery School. War Wagons can install a piece of artillery into its tower, which provides the artillery with adequate speed and the Empire army with devastating firepower.
As of today, the use of War Wagons within the battlefields of the Empire is now obsolete. However it has been known that War Wagons are refitted to serve as either an Arcane Battle Altar or a War Altar of Sigmar. In the past, War Wagons varied greatly in their size, with some War Wagons being only as large as an over-sized hay-cart, while others were the size of an Empire fortress. As of now, War Wagons have been replaced with the newer and more heavily armoured Steam Tank.
So maybe using it as a proxy for the War Altar would be the easiest.
Alternatively, as it seems to be mostly a shooting/assault unit, could it be added to a cavalry unit in much the same way as Skirmishers are added to infantry units. Only one per army? Goodness only knows what stats to give it in that case though!
Edit: Using the Warmaster unit calculator exe file that I found online a few years ago, how about this, basically less powerful than a Steam Tank:
Attacks 3/2
Hits 4
Armour 4+
Unit size 1
Min/max -/1
Points 80
Movement 20cm, cannot move into or over terrain features other than hills and bridges. May operate independently or may be added to a brigade in much the same way as a Chaos Spawn, i.e. permits a brigade to be 5 units instead of the normal limit of 4 (so an Empire infantry brigade of 4 units with skirmishers and a War Wagon might be an interesting combination). Can shoot all around. Can't be included in an Empire army if the army has a tank (or tanks) in it, to keep with the fluff of it being an old weapon, replaced by the tank.