Welcome to the board!!!

It looks like that gun is rifled 1-48 inches so I'd say get some PowerBelts and try them...I'd also try some .530 round balls and greased patches as a ball that size is plenty for deer...

Start with 75grs of real Blackpowder like Goex or Pyrodex R-S if it's a percussion and move up in 5gr increments until you find what it groups best...Anywhere from 75-100grs is plenty for deer...

Will it shoot PowerBelts??? Sure it will, the question is how will they group...

Good luck getting this done by Saturday as usually it takes several range sessions to find the best load...Hopefully one of these will group within 2 inches at 50 yards and you will be able to use her on Saturday...Good Luck...