Simple Ring Mod?

[earthtonesaudio]

See figure 88: "Low Frequency Mixer" in this appnote.

You don't necessarily need the Norton amp for this; just take a look at the schematic and you'll see the internal construction is fairly simple.  Just make two common emitter stages, tie the collector of one to the base of the other.  Call the collector-base junction the (-) input and call the base only connection the (+) input.  Add a collector resistor to the free collector.  That's a very basic discrete approximation of the LM3900. 
Then add the external components as shown in figure 88.  Finally, add signal and LFO like you would an ordinary ring modulator.

Just an thought.  I haven't tried any of this.