Hi Tony,
Lots of places on the internet have the schematics and documentation, here is one, just scroll down a bit:
http://www.radiohistoria.sk/Oldradio.../wdocu/0000304
Russian radios usually have 2.5mm mono audio jacks, you can buy plugs for it, or just replace it with a "standard" 3.5mm one.
According to the schematics the audio output is also available at the DIN-jack, as the tape output, but it is the same audio output from the final power amplifier.
As the output is coming from a quasi-complementer output stage, capable of providing up to 1W (the power limited by the 9V supply voltage), any impedance headphone, or external speaker would work.
If you use a headphone you probably want to put some resistor in series with the output (30 Ohm - 200 Ohm, depending on the headphone), so it would not overdrive the headphone.
Regards, Peter