I'd try cutting back Starline .44 Russian brass. .44 Magnum brass is going to be a lot thicker sided down around the case head; the further down you cut, the harder it gets. If you use black powder, there's not going to be as much powder space as you would get with the thinner .44 Russian brass. That might not be a problem, but the cutting will be.
I'd borrow a .44 lead bullet (probably .429) from somebody and push it thru the bore. If it slides right thru, you have a grossly undersized bullet and will probably need a custom mold. If it needs to be pushed thru, you can measure the resulting groove marks and know how to size the bullets to be loaded. Keep in mind that a slightly undersized bullet will be "bumped up" a few thousandths by the black powder, so might well shoot OK. And you could always cast some round pure lead balls a little oversized, size them to the bore, and load them up. Sometimes those give pretty good accuracy at close range.
RICs are neat guns with a lot of history to them. They were used, and copied, all over the world, even used quite a bit in our Old West.