It's good to see there are still some of these classics out sailing. Back when I was living on my Seabird yawl I used to sail in company with a friend who had an H28. As I recall
the rudder was a very simple affair with construction very typical of outboard
rudders on woodies. Basicly vertical planks held by steel
straps and gudgeons. The outline won't be too critical and can probably be approximated from old photos or drawings. You could use this opportunity to update the construction with a plywood core
and fir or spruce packing covered with epoxy/glass.
Best of luck,