It depends on two things, firstly how much you want to spend, and secondly how realistic you want them to sound.
If they are fitted with pre-2.4ghz RX18's then you can get a kit from Darkith in the US. It's a 'very' simple soldering job that takes about an hour. This converts the RX18 to a system that will have standard IR, eg Tamiya compatible. This is the cheapest option, but you will still have Heng Long sounds. This cannot be used with the 2.4ghz RX18's.
Next up is the Clark board, same size as the RX18, so all you do is take the RX18 out of its plastic case, and put the Clark board in the bottom half. It's all plug and play. You can make your own IR sensor apple to go with any board very easily.
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You will have excellent sounds, and a load of options to set the tank up how you want it to run. If you have a smartphone with an IR emitter on it, you don't need the Clark remote, you can download an IR app and use the phone to program it. If you have a Sony tv, you can use the remote from that.
IBU - Plug and play, sounds are on an SD card, pick the tank that you want from the available sound files.
Elmod - Plug and play, sounds are on an SD card, pick the tank that you want from the available sound files.
Tamiya - Not easy to install in a Heng Long tank, but it can be done. Not really worth it as the Clark/IBU/Elmod are all streets ahead in terms of what they do.
If you want to compare the different systems you might like to have a look on You Tube, there are hundreds of videos on there that demonstrate what the tanks sound like etc.