I did find out the bores were .408,and,for some reason,the factory load used .406 bulletes.(Cartridges of the World)I think 255 gr,IIRC.Likely twist will be too slow for heavy bullets.
Look to Bufffalo Arms for some sources,I believe the cases can be formed from 45-90.Likely they have dies.Starline sells good 45-90 brass.
It would not be wrong to load it with black powder.You will need to clean it properly the same day you shoot,and wash your brass.Black powder cartridge has its own set of rules to learn.It is a fun education.
Subscribe to Black Powder Cartridge News.Great magazine!You will learn a lot.
From your post,its not clear to me,is it still possible to discuss shooting the rifle,loading for it,etc,with your dad?