Hi Bob,
Sorry I don't know what the Umiak looks like, But almost any stripper design could be modified for a smaller paddler. Changing a design can be a gamble and you might want to consult with the designer. I know that the S&G designers have some smaller boats. Check out Pygmy Boats. If you have been here looking in the past you have probably seen it but, this is a boat I built for my 11 year old, I started out with a Guillemot and spent quite a bit of time at the drawing board, It's hull has an integral rudder system and the deck has been lowered for a smaller paddler. I know of one other person on this board that modified a Guillemot by changing the form spacing to make a shorter boat for his daughter. Nick describes in his book how to make a shorter boat from his Great Auk plans.