Rupp made a bike called an SS-5 that used an HS50. But I will agree that that block at least is not from a rupp becuase its white and the SS engine was black in color.
with that said though it was a 20 degree slant application engine, I dont know what the correct numbers are for the SS so to the original poster does the shroud match the block in color, or is it black? I am curious on that because I would like to know what crankshaft they used on the SS. That number you have shows the standard length 33676.
I will have HS50 parts available soon, should have extra blocks if all your other parts are good, just dont have time to mess with them right now. Next in line is an HS50 I am rebuilding for someone though so i will be pulling all that stuff out in about 1 month.