The impeller may have been downsized by the previous owner to save money which it does. So instead of up sizing the impeller, I would just down size the motor and go for a 3/4 HP full rated or 1 HP up rated 56j motor. And while you are at it, you might as well look for a two speed motor such as the B973.

http://www.aqua-man.com/row_num.asp?Ic=1177