Call Brad Schaffner at
http://www.kcsaab.com (call, don't email). He's got a lot of experience and can give you some tips. Nice guy, helpful. He's currently got a couple circle track 900s that do well; he's also done some SCCA stuff. You can do a DIY of the spring set-up he'll mention and may try to sell you -- use stock car weight jackers, narrow the lower front perches to fit very common 5-inch springs (Saab are 5.5). Let me know if you want details - I made my own.
Quaife now makes a diff for the 900. Contact Scanwest Autosport in Seattle. Ask for Rob Walden. He's campaigned two Saabs to Pro-Rally championships and, in addition to being able to get you a Quaife, he knows as much about 900 gearboxes as anyone.
http://www.scanwestauto.com
You can get a clutch diff from Sweden, but they are VERY expensive.
If you are REALLY good with electronics, and have a 16V motor, you can put a Megasquirt in the Bosch ECU box (been done a number of times). HOWEVER, (and this is where the skill comes in) you need to figure out how to use the stock MAF sensor, which has been done on other cars, but I highly doubt a Saab. MS uses a MAP sensor usually.
For a header, contact Motor Sport Service 716.665.4200.
If your car is an 88+ get cheap used 15x6 rims from a 9000.
You might find some helpful information here:
http://www.saabrally.com/forums/index.php -- these guys are club rallyers in 99s and 900s. You might also find some helpful info at
http://www.icerace.com -- there are some, uh, ice racers there running 900s. And you might find some helpful info at
http://www.specialstage.com in their Construction forum. I know, none of this is road racing-oriented, but it's better than the typical Saab-oriented forums.