There are a few things to consider. First, these are inexpensive for a reason. They are not like your typical Panasonic or Omron type devices. If you are powering these with 5V, it may be pushing the limits of what the emitter can handle. Dropping that voltage a bit might yield better results. You might want to check the actual current draw to get an idea what it is drawing.
As far as trying a different emitter, anything that physically fits will probably give reasonable results. Almost all IR phototransistors will detect over a relatively broad range of wavelengths. If they are optimized for a different wavelength, you may lose as much as 30% sensitivity, but it will likely still function at close to the same range. If you get a few in the 880nm and a few in the 940nm range and experiment, you can probably find one that will do as well. Just check that you are not exceeding the manufacturer’s max current specs.
Keep in mind that viewing angle will have a significant effect on performance. Wide angle will be more forgiving for alignment, but will have a shorter range for a given output power. Here’s a starting point if you decide to try replacing the emitter.