There are several things they could capitalize on immediately.
Remittances and charitable influx would be one.
Tourism to those who want to visit old homes and family cemeteries could yield cash ASAP. A country that legalized meth and marijuana would also attract those kinds of tourists.
Offering North Korean women as brides to rural farmers, bolstering that population and reducing desperation in their own nation, is another.
Mining takes time to develop.
Selling fishing rights or fish they catch themselves is certainly an option.
Garment manufacturing is a good fit; low cost workforce and in proximity to major shipping ports. Don’t agree with high tech manufacturing.
South Korea could profit from legal entertainment distribution.
Astronomy tourism since the whole country is really dark at night.