I've rehabbed a lot of bikes over the years. The difference is that they were whole bikes, with forks and wheels and most parts fine, and not just a questionable frame. Usually, just needed overhaul, not changing configuration. I converted my mid-low-end Dahon to 2X gearing so have the performance of a Bike Friday NWT at multiples lower cost. And when I did that, all the advice I could find on here, whether this was possible, was "No." (I had not yet discovered the Folding Bikes group.)

The labor time and effort, and parts cost to restore a Ford Pinto, is the same as restoring a '65 Ford Mustang. But it's easy to argue the Mustang is worth more. Still, if Pintos are your thing, do what makes you happy. Usually this happens with boats. I would argue that the cheap steel folder frame is more of a fungible commodity, and it would pay to start with a better canvas on which to paint.

But I have seen on here, truly lovely restorations of several Dahon Mu, down to stripping the paint and powder coating, just lovely.