If your looking for just the head, not the crankcase then heres a cheaper place:

For the NC http://www.centralhobbies.com/Engine...YS120NCEX.html

and for the SF
http://www.centralhobbies.com/Engine...YS120SFEX.html

The price for the NC head is $70 cheaper because it doesn't come with the valves. From the looks of the diagram, the heads are basically the same, so maybe you could just buy the cheaper one.