I mostly just spend the time on the new stuff. I forget the fancy arrangements quickly too. I aim to keep one or two of the fancy ones in shape for performance. Usually that's all I need.
For starters, Joseph Todaro has some good free fingerpicking ukulele tablature online at http://www.akulele.com/tablature . His arrangements are more banjoistic than most. Also, the late great John King wrote three books of top-notch finger-style ...