You could use a small FM receiver for convenience taking the output signal from the headphone connector connected to an external volume control ( about 10K ohms ) then to the decoder.
If you have something smaller in mind then the following FM receiver can be constructed with the TDA7000 IC which is still available
from some distributors . No special skill is required except care must be taken that all the capacitors all wired as close as possible to the pins. The recommended supply voltage is 4.5 volts but up to 9 volts can be used .
The coil (L1) is made of 6 turns of # 20 insulated wire on a 1/4 " form . Tuning is simple enough once you have your transmitter
operating simply tune the variable capacitor with a non-metallic screw driver very slowly until the signal is detected anywhere
on the FM band .
The signal output is very small , only suitable for crystal headphone and must be amplified if you want to hear it . The LM386
amplifier is well suited for that purpose . See LM386
