Simply of old designs is often a blessing as long as the drawing and documentation is readable and good. It can be hard to get replacement electronics for 1970s designs so sometimes you have to design new components but the functionality was relatively simple back then so it’s possible to build a 1:1 replacement

They've actually been modernized with newer power electronics and some of the soviet oversights have been addressed. They're still very reliable, and now somewhat more efficient.

Old electronics of that era could be drawn by a simple schematic and usually only performed one or 2 functions. That makes designing a drop in replacement very easy.