That book you mention sounds pretty usefull. Whats it called and can you still buy it? Does it say anything about a `89 kx250 cylinder on a `87 bottom end? (another one of my projects)
The book is called "Motocross & Off-Road Performance Handbook" and it is written by Eric Gorr. It is still available and I got it from Barnes and Noble. It gives you a complete overhaul on how to maintain and fix dirt bikes in general. In the back it lists the different dirtbikes and tells a little about each, such as: weaknesses, strengths, suggested fixes, common problems, etc. It lists each bike by year and covers suzuki, kawasaki, yamaha, and honda race bikes. It covers some trail bikes but mostly race bikes, such as the kx, kxf, rm, cr, etc. It costs about $25 retail. It doesn't always say if the cylinders are interchangeable or not. It just happened to have that listed for the kx125 you were talking about. It does not say anything about the '87 and '89 kx250 cylinders being interchangeable. Sorry.