putting in a steam stroke in a normal IC seems overly complicated. The best use of steam (or some other liquid with a low boiling point) would be in an IC-electric hybrid. A closed loop system (rankine) could power a small generator to replenish the batteries. The IC engine produces so much heat, it seems silly not to utilize a some of it.
There are other liquids that could be used other than water, Butane or neo-Pentane come to mind.
Both of those can be under pressure and still use just heat from the normal cooling system. Both can be condensed using your standard a/c condenser and you won't have to bother with working around a super hot exhaust system.
On top of that, you can use hot water off the stove to test the unit before you ever even install it in the car to verify it'll work.
If you are going to be using the exhaust to heat the liquid I'd recommend going to a muffler shop and getting an old used one and gutting it for the tubes to be run back and forth in. Putting the steam pipes around the cat may cool it to a point that it isn't as effective and wrapping them around the exhaust pipe means the system will take a while to get up to temp and might not be able to keep up with the level of heat your steam system will need. Get the copper pipes directly into the line of fire.