50g is the weight of the skein, and isn't always related to the weight of the yarn which refers to the thickness. Many yarns are put up in 50g balls, but the yardage will be different depending on how thick the yarn is. A sock yarn will have about 200 yds, a dk about 140 yds, a worsted around 100 and a bulky could have 80 yds. It looks like the original was a heavy worsted and what you have is a dk, but that could still work. Just cast on the hat and see how you like it after 3" or so.
sue- knitting heretic