In many cases, these diodes are used purely to establish a diode drop, i.e. a .6V reference (often used in pairs, thus 1.2V or so) in order to bias a transistor. If that's the case (it's been a while since I looked at a Leach schematic), then either part is way overrated, so you can use either. I believe the 4148 is a 'fast' diode which may or may not matter, depending on where it is in the circuit and your personal philosophy on fast vs. slow diodes.
Grey