First, my apologies. My schematic book does not have Iver Johnson in it for some reason. This is a first. I have always been able to look in that book and get schematics for anything. It is the gun digest book of Exploded long gun Drawings.
If anybody knows of another brand name or similar model of single shot shotgun that this may be listed under please let me know and I will see if it is in my book.
In the meantime, I did find this schematic for you:http://www.wisnersinc.com/exploded_views/IJ_Champion.htm
I also found this:
I can tell you that the Iver Johnson manufactured the Champion from 1909 to 1956. The Champion was an inexpensive firearm manufactured in both rifle and shotgun calibers, 10, 12 16, 20, 24, 28, 32 and .410 gauges and .44, .45, 12MM, and 14MM rifle calibers. Champions had a 26 - 32 inch barrel, exposed hammer auto ejector and plain pistol grip stock.
I think it would be cool to find one of the rifled barrels for it.
It looks like the value is anywhere from $150 to $300 in good shape.
Here is a pic of one in decent shape
Here is another place that you may be able to get the stock and fore end for itBoyds gun stocks
Boyd's is a good place to buy from. They offer a ton of different stocks in inventory, and are also willing to make some stocks they don't have on a regular basis. Even if they do not have one ready to go, there is a good chance they can make you one for a decent price.Richards rifle stocks
is another place that may be able to make you a new stock and fore end for the shotgun. Awesome prices and good quality stocks.
Hopefully some of this will help you out a bit.
PS: If you decide you ever want to get rid of it I'd be happy to take it off your hands.