Hi Tim,
so long as you supply the correct voltage to the transmitter it really does not matter what batteries you use – whatever you can get in. Just replace the battery pack with a new NiMh pack, you'll get the battery life back. Once Nicads start loosing their capacity its time to change. Just take care how you dispose of it, local supermarkets now take batteries for disposal.
We have planned a set-up for 12 transmitters in a row, all fed from a single master car wet battery at 12volts, and each being fed a regulated 6 volts from it via a jack plug.
2600 mAh will give far longer between charges than 1400 mAh, believe me. Can you get LiMh batteries?
Kim