Plain wheelweight metal or something similar would probably work fine if you're just looking for ammo to fire.
Personally I would be looking to replicate the ammo its designed for with the caveat that it needs to be fairly snug in the throats with lead bullets for best results. I'd treat it like a Colt 1911, a broomhandle or Tokarev and run as close to jacketed hardness as I could make, as tight as will function when fouled, but that's what "I" would do. .001 or .002 over the jacketed bullet to start.
Autos of that era, at least the handguns, had very shallow rifling and required jacketed bullets despite relatively low pressure levels. Dead soft lead bullets would strip engagement in them and generally needed backed off until they didn't, sometimes to the point of function issues. I don't know if that's the case with the Winchester SL's, but it would not surprise me.
The powder coated bullets might be the best option in this particular case, since they are essentially a "jacketed" bullet and would still be soft enough to punk up a bit if you choose to hunt with it and be more forgiving in regards to fit and fouling, perhaps feeding as well. Another default is a gas checked design, which will keep the bore in constant condition regardless of alloy hardness, but they generally require more care when sizing and seating the check if you want accuracy.
