If you want to get performance similar to the Merco using electric power you will have to go brushless. We have done this successfully with the 46" Crash Tender and the details are on this website:-
To see the boat in both static and action pictures (well up on the plane) go to the Photo Gallery and select Fire Boats. Go to the entry by mrpthesailor dated 13/11/08 on page 2.
To check out the power set up go to Your Power Setup section and scroll down to the entry by mrpthesailor dated 13/11/2008. Here is a link to the power setup:-
http://modelfireboats.com/rc/setups.php
A power setup like this does not come cheaply but the results can be Impressive. Duration will depend om how you sail the boat, with realistic cruising mixed with high speed runs should give about 15 mins.