Elektronika MK-52: The MK-61 big brother?

[jebem]

The VFD display is available as a spare part from Ukraine, the country where those calculators were made during USSR era and after that a few years more.

eBay lists several sellers, some offering them at around one to three USD each plus s&H. I bought some of those a couple of years ago and paid around four USD for shipping.
Look for "ILC2-12/8L VFD Display Nixie Tube for Calculators Electronika MK-52 MK-61" and you will find several offering.

The keyboard will not have any positive action when pressing the keys, no matter what kind of foam I used. This seems to be a characteristic feature of these MK machines.

To clean the PCB as well the enclosure parts I use warm water and dish detergent, and a soft brush.
Let it dry for 24 hours, preferably close to a window where it can get some mild heat, but avoid to expose the plastics of the enclosure to direct sun.

After that the PCB receives a proper cleaning with IPA applied with a soft brush.
I never use IPA on the cover, as the labels may be erased.

I never use acetone or other aggressive chemical reagents on electronics like these, so I can't comment on that.

Edit:
For IPA, see https://en.m.wikipedia.org/wiki/Isopropyl_alcohol
It is easy to find it in plastic bottles, 1000ml or 2000ml, in hardware stores.