It looks like there are two I/R LEDs in series to me (LED 1 & 2)? A bit hard to tell for certain from the diagrams.

I would be replacing both at the same time - Jacar part # ZD-1945 (blue lens) for the transmitter @ $1.25 each and the receiver diodes also seem to be in series - again I would replace both, Jaycar # ZD-1950 (clear lens) $1.50 each