I converted the Trumpeter 1/35 scale LCM-3 to RC a couple of years ago. I chose the Trumpy kit because I read in a review that the Trumpy kit's hull is a little deeper, so I figured it had more room for RC gear. Here's the link to the comparison review on the IPMS Perth website:
http://www.perthmilitarymodelling.com/reviews/vehicles/misc/lcm3/lcm3.htmI powered my model with a pair of disassembled standard servos. They provide plently of power. I used the stock kit rudders, and a pair of brass 25mm props. I'm not entirely sure, but I think I used those short 2mm prop shaft/stuffing tube assemblies available from Billings. For the motor to shaft connection, I used a ball and socket universal set designed for use in HO scale model locomotives. (The best universal joints I've ever used for a servo powered model!)
My attitude is that plastic model conversions are somewhat disposable, so I just build the battery into the model. I figure that, by the time the battery wears out, I'll either be sick of the model or it will be pretty worn out and beat up from years of use. (I got 15 years of run time out of my Lindberg tug!) In this case, I used a 4.8 volt NiMH 2300MAh battery to power the radio and servos. The battery fit nicely under the cargo deck.
A mini servo handles the rudder, and the two motor servos are connected to the throttle with a Y cable. (I'm using a cheap two channel radio.)
Here's a link to a picture of my model underway. It it carrying a Sherman tank, just to add some weight up forward.
When I orginally built the model, I left the bow ramp free to move. My orginal plan was to rig up a catch so that full rudder, plus full trim, would release the ramp and let it fall open. (I never got around to setting this up.)