I think you mean JCM 800.
I haven't seen 900s in kit form.
I went through your deal a few years ago.
I read everything I could find, and looked at Layouts and schematics galore.
If you can solder and read layouts, and schematics, you're well on your way.
The important thing is to learn how to work with and around the high voltage.
How to discharge Caps, and how to know if the circuit is safe to work on.
This site has a section on Electric Safety. At the top right.
Home Index Page
Here are some of the sites with layouts.
CeriaTone.Com - DIY Guitar Tube Amp
TUBE AMP KITS - SALE! JCM800, 18WTMB, 5E3, NEW PRICE 5F1
Good Luck, and be safe!