Hello and welcome to the forum.
To calibrate a device, you need precision instruments. Have you got some ?
An analog oscilloscope is not a high precision measuring device, it is very rare that it requires calibration.
If you are not a specialist, do not touch the adjustments, you risk doing worse than it is.
If it works, use it as it is.
If something is totally wrong, it's probably a failure, not a calibration problem.
For plastic parts of Philips oscilloscopes, it is true that they become brittle like glass .... No solutions for that because these parts are not manufactured for many years ....
You can always try to ask for parts on the Dutch forum :
https://www.circuitsonline.net/Philips oscilloscopes were manufactured in Holland, (Eindhoven) and it is then easier to find parts in this country.