A motormaul is just a power supply for your amplifier.
It is just like a linear base-station power supply without the AC input section, only the DC regulator "pass transistor" output. It also contains switching to allow you to recharge the batteries. Some do this switchover to charging batteries automatically when you receive.
The regulator part, which powers the amplifier, (LM723 circuit) an example can be found athttp://agurk.dk/bjarke/Projects/PowerSupplyFET/13.8V%2040A%20Power%20Supply%20Schematic%20Prints.pdf
Take off the AC input transformer, bridge rectifier, and the big caps on that bridge, and you have the regulator output part of a motormaul. You just need to remove all that 15v over-voltage protection because you need to go up to 18v or more on output.
The main pass transistors need to be scaled to the amount of current your running.
that four-IRF150 example is good for 40 amps.
The input to the regulator is two 12v batteries in series for 24v. When you switch over to charge (radio in RX), all the batteries need to switch-over to all in parallel to be charged by the 13.8v from your alternator.