Unfortunately I did this mod a while back and didn't take any photos while doing it. So this "tutorial" is mostly just proof that this mod works.
What you need:
-Vintage MT500 radio
-3V battery holder (2x AA or AAA batteries)
The gutting of the MT500 is pretty simple, just take the MT500 apart leaving just the speaker and switches.
All you need from the spirit radio is the tiny sound board.
Hook up the "talk" switch from the spirit board to the talk switch on the MT500, and the on/of switch to the MT500 on/off switch. Run the speaker wires from the spirit sound board to the original MT500 speaker. Run the power to the sound board to the battery holder which you can velcro or glue into MT500's battery compartment.
Put it all back together and your done ! It should sound even better/louder than the spirit version as the speaker in the real MT500 radio is much higher quality.
The added benefit of this mod is it makes your MT500 much lighter with all the guts removed.