You can replace it with a high impedance earphone, or you can use an impedance matching transformer to match the radio to your earphone.
An impedance matching transformer has one side with lots of turns on its coil, and another with fewer. You connect the side with lots of turns (an impedance of 1000 or 2000 ohms) to the radio in place of the earphone.
You then connect the earphone to the side with fewer turns (between 4 ohms and 32 ohms).
If space is a problem, you can try winding the antenna around the room in a big coil, but having the antenna outdoors, and as high up as you can get it, will be best.
A cold water pipe is usually used, since one end of it is usually buried (not so for hot water pipes). Yours may be plastic where it is buried, which would not be good.
You may not have a good ground connection. Try finding some metal that is buried at one end, and connect the ground of the radio to that.
If you have access to an amplifier, such as a stereo system with a phonograph input, try connecting the phonograph input of the amplifier instead of the earphones, so you can amplify any signal you get.
If you still get no signal, then the problem is NOT your earphone or your antenna, and may be your connections or your diode.
If you are having trouble finding good diodes, try using any two wires from a three wire transistor.