MIDI Out... is it difficult?

[sk-1]

I've seen a lot of retro-fitted keyboards and toys with MIDI In and MIDI Thru... but I am wondering, just how difficult is it to wire something up so that it becomes a MIDI controller.

Say... an SK-1 or an SA-5 with MIDI Out.  Can anyone tell me just what is involved in such a setup and why I've never seen it done before?  Is it really that hard?