Most radios should be able to use these modules, even pretty ancient ones. Older sets tended to use the NE5044 encoder chip (mid '70's onwards), which pushed down the cost of multichannel radios by providing a 7-channel encoder in a chip format, as opposed to discrete circuitry used in earlier sets.
Some companies produced their own chips, I think Futaba were one, but generally they're easy to convert too as the tuner boards are almost always separate from the encoder section, so it's fairly easy to find the ppm pulse and pick up a a pair of pins from +ve and ground.
The cased modules can be taken apart leaving you with a board which can be mounted inside the case if the set isn't modular. Flyleads can be soldered to the input connector, and the aerail mounted somewhere suitable on the case, perhaps replacing the TX original.
If the set has a buddy box/trainer port it's even easier as the connections needed are all there, and information on pinouts can usually be found fairly easily online.
If you have a transmitter with extended channels that doesn't conform to PPM standard e.g. Robbe F-series or Graunmer MC-series, then you may have issues with the extra functions. These cheat a bit to get the extra functions within the 20ms frame rate of PPM. The way these newer systems work, they don't like it, and tend to play up, so be aware of that if you're thinking of converting one of those sets.