#82223 removed fields from the index pattern saved object (except for scripted fields).

x-pack/plugins/reporting/server/export_types/csv/generate_csv/index.ts:86 loads the index pattern saved object, looks at the field list and loads all the field formatters.

pseudo code recommendation -

const fields (assume defined);
const indexPattern = await indexPatterns.get(id);
fields.reduce((collector, field) => {
  const indexPatternField = indexPattern.fields.getByName(field);
  collector[field] = indexPattern.getFormatterForField(indexPatternField);
  return collector;
}, {});

Its possible that the majority of the work will involve bootstraping the index patterns service.