A 44-40 case and a Lyman/Ideal 454-485 gas check bullet works well in an original bore and chamber. The 44-40 case becomes the gas check, fitting into the GC recess of the bullet. This still requires some sort of CF conversion.
    Previously,  I mentioned chambering to 45 Long Colt. The .454 bullets work well with the original .46 RF bore and brass is readily available. The original chambering can be opened up to .45LC.