I've modded most if not all of my bikes, too.. for a cheap bike, stuff like seat, grips, stems that allow the bike to be better suited for my riding position and comfort.
Gearing and other performance mods would come once I wear out the OEM items, just to keep the cost of bike down and less waste of unwanted parts, even though I donate plenty of bikes/parts to local Bike Kitchen/Church for rehab bikes.
Origin of bicycles all start from being more efficient, whether you want something to allow more practical transportation than walking or all out racing, and to each purpose, selecting the right tool for the right job simply allows the rider to complete the task easier. Depending on your task, you mod your bike to compromise one way or another.
When OEM gearing is drastically changed; handling and operating conditions change for the bike, it may or may not be out of the range for what the bike is capable.. but on sub $100 folding, I'm getting myself familiar with the bike before I use it outside of its designated operating range from the factory.
Before investing more money/labor into a cheap folding bike, why not ride it as is until something falls apart use it to its factory capability to the max.
Save yourself some money and learn to operate it at its capacity.