Cleanup for black powder is a different breed of cat, as bigsky indicated. I use boiling water and ivory liquid for muzzleloaders, finishing up with Rem Oil and 3-in-One. A lever action would be a bear to clean that way, though you could possibly use a flexible, non-metallic cleaning rod fed from the breach, drawing up the hot water and ivory liquid from a bucket with the muzzle end stuck in the bucket. Very messy ordeal, and you will burn your hand on the hot barrel if you're not careful.
If you can find brass and dies, it may be possible to load your own smokeless cartridges. Don't do this if you can't find reloading info on this cartridge in a reloading manual. IMR 3031 is a good smokeless rifle powder because it burns slower and doesn't generate excessive chamber pressures, i.e. relatively safe to use for old rifles. Always start off with the minimum load. When I reload I usually stay with the minimum load.
I don't know whether it is safe to use copper jacketed bullets with black powder loads. Maybe someone here knows. If jacketed bullets are not recommended, and you can't find lead ones, you may have to buy a bullet mold and lead smelter...again, more work. I would advise against loading black powder in cartridges the way modern reloads are done. Black powder has a reputation for being tempremental, and can and will touch off by percussion. If this happened while reloading, or if one cooked off in the tube magazine of the rifle, you can imagine what would happen.
Sounds like you may have a nice old lever action Modas, which probably has collectors value. The guys here have good advice. I'd be interested in what the gunsmith says. The ideal situation would be to find factory ammo ready made. Even then, I wouldn't fire it more than a few times, to avoid taking a chance on damaging the rifle.
Les