To clarify, the bottom bracket is the bearing assembly that attaches to the inside or outside of the frame (yours is outside, so external). And the the crankset is the crankarms/chainrings, so they are 2 different things, just in that link you gave they grouped them together since they work as one system. You should be able to keep your bottom bracket since they standardized much of the external bottom bracket systems, and should be able to fit a shimano to the bike. But you could take the bike down to your LBS to see if your bottom bracket would fit a shimano crank.