There are a couple of the boutique ammo makers like Buffalo Bore and Underwood which still make the .38-44 loads much like the originals, and duplication loads are easily handloaded. The major ammo manufacturers (Federal, Remington, and Winchester) no longer load the old .38-44. I believe they stopped in the mid-1970s. The older reloading manuals have recipes, 2400 was the usual propellant given for doing that back then. There may be better propellants available today for the .38-44, but probably not by much. Today's +P .38 Special loading is not the same as the .38-44. From the major manufacturers, it uses a lighter bullet at a lower MV. You might say correctly that the +P loading replaced the .38-44, but by no means does it duplicate it. The most common bullet used with the original loads was a 158 grain metal capped bullet. It looked like a FMJ, but the bore bearing surface was lead. Presumably, that gave it better performance in penetrating auto bodies. The .38-44 load was developed mainly for that purpose.
The old factory ballistics tables show MVs in the low to mid- 1100 ft/sec range using 150 or 158 grain lead bullets, but of course that will be different from different revolvers and barrel lengths. I don't know the peak chamber pressure of the old .38-44 loads, but I used the QuickLOAD computer program to estimate it, and came up with something in the 25,000 PSI neighborhood.