What capacitor?

[Pagan]

Hi I hope you can help me. This is my first time on this forum so I hope I've posted in the correct place.
I'm relatively new to to dark art of electronics.
After my detector( the pw seekit) that I built while at college I the late 70s was damaged beyond repair last week by a leaking workshop roof, I've decided to build a new detector, I've decided on a induction balance design from the 80s,as like the seekit it's simple,
The problem I have is the substitution of polystyrene capacitors, while they are available, there not available locally ( like to support local traders).
What would be a good substitute, mylar? Ceramic? Or tant.?
An ex electronic engineer told me its not the material that maters its the value and voltage, I'm not convinced, what's your recommendations??