www.introspectiv.org has some interesting info on building a 9090 clone.

9090 User Guide | pdf 74kb | date : 16/11/02

9090 Parts list | pdf 29kb | date : 5/Jan/04

9090 Wiring guide | pdf 86kb | date : 14/10/02

9090 Board layouts | pdf 237kb | date : 14/10/02

9090 Schematics | pdf 203kb | date : 10/6/02

There is a complete part list plus board layouts and useful links to other clone sites. It does look to be fairly complicated but the interesting part is the schematics for each individual drum sound.