What is the cost of retrofitting older steel?

I have a general interest question with a focus on older European steel loopers (Pinfari, etc.). Over the past few weeks, I’ve been doing a deep dive on classic, old steel models and their maintenance costs/records.

Purely from an engineering perspective, how difficult is it for smaller, independent properties to retrofit these legacy European layouts with modern control systems and safety PLCs. Obviously, they are pennies on the dollar compared to modern custom fab, but does the mystery hidden installation coupled with possible foundation work normally negate those savings and put them out of reach for a small park?

Looking to hear from someone who has worked on older steel.

Do you mean the portables? I imagine you don't need a particular foundation since the load is carried evenly throughout the structure (they tend to sit on a steel grid). I also imagine that any of the aging rides have already had the control systems replaced at least once.