Hi,

Trust me its not worth the effort / expense to change the gearing.
Learn to spin on it going fast. I'm 50+ and can do 18mph on one.

I broke a plastic brake lever letting the bike fall over, amazoned/ebayed
an alloy replacement pair, they where cleap, main housing still plastic.

Fit some good brake pads to the front, keep the old ones as spares.

A pair of nice rolling tyres will improve your average speed.

A pair of combined ergonomic grips with bar ends is an idea.

Fenders are a good option.

I bought a folder a Christmas to get back into biking.
At Easter I bought a road bike to go faster and further.

rgds, sreten.

FWIW when I first got the folder I was very slow on it.
Now I'm not, but will never be quick like the road bike.

$120 updating perfectly usable parts on a folder versus
spending say $200 on a nice used road bike, no contest.