What do you consider "more affordable"? I don't think $400.00 for an uncommon gun that is out of production for many years now is all that much to pay.
In any case, the solution is relatively simple: find a muzzleloader in a shop somewhere that meets your price requirements, buy it, get the hacksaw out and cut it to length. Depending on what you are modifying, you might need to shorten the stock and/or the under-barrel rib... which might mean some drilling and tapping or maybe JB Weld work. Next you need to re-crown the barrel and file a dovetail for your front sight. Personally, I would get the gunsmith to re-crown the barrel, but you don't really have to do that.
You just might end up with a functional "canoe gun" for less than a Benjamin!