[Arran]

Electrically most circuits don't care if you use a non polarized capacitor in place of an electrolytic capacitor, the only things that matter are the voltage ratings and the capacity in microfarads. Since polyester film capacitors are relatively compact and inexpensive there is an advantage in using them in some places over an electrolytic capacitor when you can. At the time these sets were made non polarized caps meant paper and foil, which meant bulk, an electrolytic capacitor was substantially cheaper and more compact for the same capacity and voltage. Good quality electrolytic capacitors have maybe a 30-40 year lifespan, China crap brand caps last maybe 5 if you are lucky, polyester caps could last indefinitely barring manufacturing flaws, overvolting, or excessive heat.
Regards
Arran