Assuming that you're dealing with folk music, you shouldn't have to use accidentals much. You just need to be in the right mode: for minor key, instead of starting with doh, your scale starts with la, for instance. The intervals between the named notes don't change.

The Christian Harmony school of shape note singing uses a seven-shape system like your solfa. Sacred harp uses four shapes, because that covers all the needed intervals.

~ Becky in Tucson