I took off the fd and left the triple front crankset on with the chain on the center gear. I had to limit the rd to the lower (big) four gears to keep the chain from rubbing on the large chainring.
On my bike, I ran it 1X8 with a single speed cranket and could run thru all eight gears with no trouble. My bottom bracket was 3mm short of the recommended spindle and it lined up dead in the center of the cassette.
I don't know if your bike has removable gears or not. A lot of inexpensive cranksets a riveted or something together, anyway they don't come apart. You will just have to try. You might have to run you choice of gear inside the crank arm to line up or maybe change parts.
Unless someone has worked on the same model bike as yours, with these questions in mind, it would be up to guess and trial.
Good luck. Whatever it takes it should be minimal and work fine in the end.