You probably could take a 5/8 wave 2 meter ham antenna and cut it down to a 1/4
wavelength
antenna for use on 123 MHz.


There's more to it than that. The impedance has to match. For a 1/4 wave
antenna, all you need is 1/4 wave worth of metal. But for a 5/8 wave, you need
a coil at the bottom too, so that the antenna looks like 1/4 wave to the
transmitter, and works like a 5/8 wave to the ether. A ham band (or any other
band) 5/8 wave antenna will already have the appropriate coil, but if you
change the length significantly, the coil will no longer be the right value.

For 123 MHz the difference is probably not all that much, but it might be
enough, depending on your transmitter.

Jose

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