Without a schematic is impossible to suggest you to replace some component, however perhaps some kind of help is possible.
Some question:
1. what about the range you reached at present ?
2. are you using an antenna ?
3. what about supply voltage ?
4. what about the range you need and is it outdoor or indoor ?
5. are there obstacle (any kind: walls, trees, etc) in between TX and RX ?
since the actual range depends from both RX and TX antenna gain, you can increase the range just using a long wire (roughly 23.8cm are 1/4 wavelength, however you can try to use longer wires)