Welcome to the club! I have four Echolettes. I don't think that you will find anything "comprehensive". The manual does not have much technical information other than the schematic and you have already found that.
Here is a place that can rebuild the rubber roller: Terry's Rubber Pinch Rollers & Wheels
I do not know of a source for heads. You can buy tape loops on ebay or just make your own.
These are very difficult to work on since the chassis is very crowded with parts. I have replaced all of the caps on two of mine to reduce some distortion and it is not an easy job. I still do not have what I could consider a properly working unit.
Here's a pic: