There a couple of options you have to make the 2.4 camera tilt work:
1.) Find a higher torque servo and adapt it to the 2.4 E-chassis. I'm not aware of anyone who's done this, though there are probably some people who have. Perhaps they'll post here.
2.) Get the laser cut files for a 2.5 kit, and cut yourself the parts for a 2.5 E-chassis. Then mount the parts for your 2.4 kit on the 2.5 chassis. Colin Ho built up an OpenROV this way and it turned out well.
3.) The easiest way is probably to modify your 2.4 e-chassis to work like the 2.5 e-chassis. See this forum post. In fact, this ended up being the genesis of the 2.5 E-chassis design. I finished up this ROV (perhaps I should update the thread with more photos), and the general design works just fine. I'm in the process of modifying another 2.4 kit the same way.
A couple of caveats about the modification-
- Since the servo is flipped around from the stock configuration, the stock software works backwards, i.e. commanding the camera to tilt up tilts it down instead.
- The HS-81 servo included in the kit has plastic gears that are very easy to strip if you ever jam the camera stage, which is quite easy to do while playing around with things. The HS-82MG is the metal gear equivalent of the HS-81, and is highly recommended here.
Hope this helps. Post some pictures of whatever you end up doing for your ROV.