Looking through the Specs it would appear that the Jib and Spinnaker Dimensions for a J-24 are extremely close to the Compac 23. Check it out for yourself:
Compac 23 Specs: http://sailboatdata.com/viewrecord.asp?class_id=232
J24 Specs: http://sailboatdata.com/viewrecord.asp?class_id=245