Hello Bruce,
You could do this as a starting point. Assume that the foil loading between the CB and rudder are in the same proportions on the I17 and I20. If this true, then a starting point on calculating correct CB and rudder size for the I17 is to take the I20 CB and rudder areas and scale them down by the ratio of the I17 sail area divided by the I20 sail area. By varying mast rake, you can shift load from CB to rudder or from rudder to CB.
Bill