If you can build a kit, you can hack out an underhammer. Find a barrel on sale for under a hundred bucks (GM) and get your scrap together. Buy the nipple and breech plug and make the rest. I bet you can do it for under 150 bucks. A muzzleloader doesnt get any simpler than this. Truly an elegant design, ie;simple and effective.
The H&A is brilliant, 3 moving parts!
One modification that is worthy would be a patent breach, color cased. installing a nipple in a barrel is effective but maybe not the best option for strength and corrosion.
Ed