That extractor looks very dodgy; especially the extra "pad" welded on. I'm guessing the this was to alter extractor travel for .22RF. In fact, the whole extractor looks very rough in both shape and surface qualty. See if "Numrich / Gun Parts" has any good originals.

The other thing that seems odd is the "non-appearance" of the distinctive "teardrop" shaped cocking indicator; has it been replaced by a "custom" component as pary of the conversion to .22?

.45_70 is a good choice; alternately, .444 Marlin. How about .38-55 / .375 Win?

You may also consider going with .303, or, more practically in terms of bullet selection, .30-40 Kragicon.

Usual caveats apply: only use genuine Brit. ordnance-made actions in GOOD condition. "Khyber Pass Specials" could be a bit "exciting" in anything hotter than black-powder loaded .303.