All the Rx's I have come across are all pretty similar in size, especially the FM receivers. I'm sure you can get some pretty tiny ESCs, like teh ES-20, and Great planes have some ESC's that don't have a case, just heat shrink wrapped around them. Is the SPY much smaller than the ES-01? I've never seen one, I'm just going on what's on the tower website..
The smallest servo I have seen has to be the Mini X servo, but I'm sure there are more out there. Some of the sailplane servos are
tiny !!
I'm actually thinking of decasing the Rx and ESc (the CPR kit) I got with the Kawada, and I want to put them in mini Z chassis. I'm gonna use the steering setup from the kawada with a miniz bulkhead, so I have adjustable toe. My rear (kawada) diff nefer worked, and I have been using a miniZ stock diff, with the brass extension piece screwed on the end. (needless to say it's crap..).
I suppose I could just make a custom H-plate for the mini-z Diff, but I dont have the tools. This way I will have the bset of both worlds, if it all works out...