not sure, need to find out if the motor cradle will accept those motors, or if the 947 cradles will line up to the mounts already in there, or like you say, just need new mounts located where the need to,...
you can't even put 787's into 650-720 engined boats. not without fabricating new motor mount spots in the glass inner hull. then you have to re-do everything. buy a challenger with the bigger motors. sell the speedster to someone who will think it is fast enough. save yourself a buncha money-time and headaches. not to mention the end result will not have any value for re-sale.