Cheapest way to make 8 speed folding bike a good climber
I've done something similar with a 6 speed bike  (42 tooth chainring, and 14-28 rear.

Its now a 48/38/28 with an 8 speed 11-32 cassette, so the lowest climbing gear is 15.4 gear-inches.

A folding bike is kinda bad for touring for multiple reasons


Small wheels make everything harder.  Mine's got 20" wheels, some are 16" and some might be as large as 24"  Small wheels make the chainstay shorter than full sized bikes, so you can't use rear panniers.  Also, if I have a load on my rear carrier I get heel-strike.   There's little space on the front forks for panniers, and anything you did fit would interfere with folding the bike.
Weight balance is dreadful on a climb.  Mine's got a super-long seatpost, so when on a decent climb you have to be out of the saddle, else it gets very lifty.
Weight balance is bad then too - I've had the rear wheel simply loose traction on climbs, mostly when I've put a bit of extra push on it to get up and over a lip, or a bit of loose gravel.
Pedals - if your folder has folding pedals with plastic, either change them for normal pedals or get some high quality MKS folding/clipout ones.  Mine had plastic shells and a plastic tang on the locking mechanism, so the only way they held load was when pressure was put on the stubby bit right by the crank/thread.
Gearing - if you shrink the single chainring, then you have lost top-end gears.  It is a horrid feeling when riding in the highest gear you have, and you're still spinning out and bouncing in the saddle.  


SOLUTION: Fit at least a compact double chainring.  Or do what I did and fit a spare triple chainring from some old MTB.  The main downside is there's nowhere on my frame to attach any kind of front derailleur, so changing is a finger-dirtying shift with your right hand.  Also, big/big gives a bad chainline which can cause the chain to drop to the middle chainring on bounces.

You could also explore a new BB like a schlumph mountain drive, which is a 2 speed IGH in the front chainring.  Downside, it will be expensive.