The range of my design can be up to 5 or 6 feet, but it depends greatly on many factors, including how electrically noisy the environment is, how sophisticated the software algorithm is that reads the analog signal, as well as the orientation of the pickup inductor, L1, to the chest transmitter. The magnetic field generated by the chest transmitter is very directional, so if it is held perpendicular to L1 the signal is weaker and the range shorter.

I don't have a particular part number to recommend, but it's important to understand that inductor L1, along with capacitors (C1A-C), form a 5kHz resonator. L1 is the magnetic pickup, so its physical properties will also determine its effectiveness. What seems to work well is an inductor wound on a ferrite core and longer cores seem to be more sensitive. In the last design I used a 6.8mH ferrite core "cotton-reel" inductor, but I've used other inductors as well. You can use a different value, but it's important that the resonator circuit stay tuned to 5kHz (or what ever the frequency of your HRM transmitter is). The resonate frequency can be calculated as follows:

    f = sqroot(1/(((2pi)^2)LC))

    f = sqroot(1/(((2pi)^2)LC))
      = sqroot(1/(39.5 * 6.8mH * 147nF))
      = sqroot(1/(39.5 * 10^-9)) Hz
      = sqroot(25.3 * 10^6) Hz
      = 5.03 kHz

You can purchase the MAX4169 directly from Maxim's online store. The MAX4169 has good rail-to-rail performance and decent speed, but there's nothing particularly critical about using it. There are plenty of other quad opamps that should work fine.

Mouser, as well as others, carry the CDSH270 diode. It's a small signal (100mA) Schottky diode, and just about any other small signal Schottky diode could be substituted.