Some time ago Jim sold plans for the S51. You should also be aware that the kit airframes are dimensionally somewhat different from what was specified in the early plans. So, for example, a plans built fuselage cannot simply be bolted to a kitbuilt wing.
However there are a number of plans builts S51s in various stages of construction, and at least one has flown. In fact, much of the production tooling for the kit came from a plans builder. If you are a plans builder in need of help, I suggest that you talk to Mike Archer. Mike produces parts for the Thorpe 18. He has a plans built S51 underway, and reportedly has as complete a set of drawings as exists. You can contact him at S18Thorp@lightspeed.net, or by telephone or FAX at (559) 539-2755. Alternately, you can try MarK Smith, firstname.lastname@example.org, who also has a fairly complete set of plans.