The large chainring is swaged permanently to the crank arm and can't be removed or replaced. The smaller ring can be unbolted and replaced.

Basically, this is a very low-line crank and the nutted bottom bracket further indicates this entire thing was pretty much bottom of the product line. If you want to use this cranks as a single chainring, you've got to use the existing big ring as is.

You could replace it with any JIS square taper crank if (and this is a big if) the current spindle is the correct length. Frankly, I'd get a complete new crank and matching bottom bracket.