Not much help but not a very common cal and gun so what the hey.... I do have three 45-75s in the 1876. I baby them. The 1876 design has its limitations for pressures... conservative loading is the way to go.
I have NO idea about Chaparral bore specs, twist, etc. If they kept to original (and that's a big if!) then the 50-95 was considered an "Express" type cartridge. Which means a lighter weight but faster bullet. I'd for sure consider the lighter bullet. IIRC Mike V. as per his excellent book on leverguns, found that bullet weights in the 300-320 gr. range worked best in the 50-95 he shot. Again, may depend on the twist in the Chaparral?
Trailboss is a really good low velocity/cast bullet type powder for high expansion ratio pistol cartridges like the 44-40, 38-40, 45 LC, 38 Spc for Cowboy action shooting. Powders like 5744 are much better suited to the big straight-walled rifle cartridges.
You really don't need load data for blackpowder cartridges if loaded with REAL blackpowder. Just because the 50-95 has the "95" number in its designation, it means little as far as the best BP load in your brass with your bullet. Determine the best bullet seating depth for working in your action. Then fill the case up with something like 1.5 Fg or 2 Fg black to the base of the bullet. You should be able to cut some kind of card wad (milk carton, cereal box, or something paper based and dense) with a sharpened 1/2 inch pipe so the wad fits snugly (just a friction fit) into the case. By adding the wad (somewhere between .020 and .075" thick) that should give a little compression to the powder. LIGHTLY crimp the bullet. Try to use a BP type soft lube for the bullet. Bore butter or other muzzleloader lube will work in a pinch... just finger apply it to the bullet. A mix of beeswax and veg oil or Crisco will work. Powders like Pyrodex will work BUT most of the substitutes will yield higher pressures... not needed in that cartridge, in that gun.... certainly not needed for deer. Accuracy is everything.
Also, slug the bore and size the bullet to groove diameter or maybe .001" larger than groove and use a soft alloy for the BP. I don't know what hardness your wheelweights are??? There is no standard hardness for them as far as I know. If they have a BHN hardness of about 8-9 then should work fine. If your alloy is much harder then mix with pure lead to soften a bit. Clean the bore often.
