Well let's see, First of all you need to check the unused bands on your FM radio, will be very useful if you live outisde the city, as many local bands will be free at your place. Choose your central frequency to be in the middle of the band you notice to be free.
Second, you need a low power transmitter.
Third, the bandwidth of your channel must be about 20KHz to give you the whole sound quality of your MP3 player, with left and right channels.
Those are the main needs in your circuit I suppose.
I'll try getting back to you if I find anything more.
Salam.