I'm not aware of any chainplate or rigging issues with 21's - they're pretty solid boats. There are 2 internal layouts - early ones had the cooker/sink sliding out under the companionway, later ones had a rather better cooker/sink unit to the left of the keel box. And although the outboard well was fine for 5hp 2-strokes, it's a tight fit for equivalent 4-strokes.
Good luck!