Personally I've had good results using a DID VX chain coupled with front and rear Afam steel sprockets- I know you can save a few grams using alloy but on a road bike (unless you are really going for a weight saving exercise like Uptopia) they don't last as long generally.
Everyone has their favourite supplier but I like Bike Torque Racing and have used them from way back when they only did parts for GSX-R's. They will even supply sprockets in alternative sizes if you want to mess around with gearing. I've gone up on the rear which makes sixth much more usable in my opinion without making it too 'buzzy' while cruising.
https://www.ebay.co.uk/itm/Ducati-90...AAAOSwlkpcPe1C
Air filter wise I have used a K&N with an open air box and a stage two Dyno-jet kit for the (almost) twenty years I've owned my 1997 900 without any issues.
As others have surely said I would give Luke or Craig at Moto Rapido a call for all your service and original part requirements and in my experience it is worth checking out those costs before presuming the likes of eBay would be cheaper.
So depending on the indicators you like it could be that original style are cheaper from them or you have the Evotech type for a more modern look while maintaining a level of Ducati-ness:
https://www.ebay.co.uk/itm/EVOTECH-I...gAAOSw44BYSrny
Or there are older style Ducati Performance versions which are in period to the 900 model years but they, like most other period parts are getting rarer or going for silly money.