If the current Philips HID-PrimaVision PV C 70 /S CDM electronic HID ballast of my tracklight would fail, which brand and model of 230-240V 70W electronic HID ballast are good and reliable and would fit the ballast case of my tracklight?
Philips, Osram, Tridonic, etc...
See attachment for size of the ballast case of my tracklight.

A common sense law of keeping yourself sane says solve the problems in the order they do appear. So your ballast is working, why bother? )
These beasts are rather reliable, especially when used intermittently with low hours.

Having said that, all the brands you just mentioned are just great. Osram probably the most polished, Tridonic the most advanced and interesting schematically.
Plus Vossloh-Schwabe and also Chinese Venture branded ballasts are good too.

There is no shortage of 70W electronic MH ballasts in the world now. As people dismantle their MH lighting systems, there is a LOT of such ballasts floating around second hand for some laughable money. Both NOS and used. Used ones with heavy hours likely having their capacitors close to EOL, but most still work anyway, albeit with slightly increased consumption.

Now please do not offload your responsibilities to the fellow members, go google some datasheets with mechanical drawings and verify if the ballasts will fit. I bet most of recent ones will do...

Here is a datasheet for one of the most ubiquitous Osram MH ballast ever!

This one for a great Tridonic C011 series

I've just noticed that the prices of electronic HID ballasts, rising.

No I do not agree. For the markets I know (Kazakhstan, Russia, Germany) bread and butter types 35 and 70W internal are available for almost free giveaway. Others, 150-250W electronic, 20W electronic, are more pricey, but still can be digged out for some reasonable money.

Recently we found a cool 250W outdoor rated DALI-controlled (!!) Osram ballast for about $35, that's kinda 1/10 of the original price I think.

Is this Osram ballast would be fit in the ballast case of my tracklight?

May be no. This is a rare version with an external plastic case with some specific connectors. You need an usual plain sheet metal cased one.