It depends on how much you want to spend on it.
Usally fast forward and reverse needs to be lubricated when moving gets slow.
The drivebelts needs to be fresh too and if they are still the originals mounted I would highly recommend to get in touch with Dillen here.
Opening a Beocord 9000 is a very simple thing to do. Losen the 2 screws, one on each right and left side on the back of the Beocord, and open the top.
The drive unit is hold with 3 screws and 2 wire plugs which is also simpel to remove.
When the unit is out you can lubricate the unit and mount the new drivebelts.
The electronics of Beocords are pretty long lasting in general and as they are pretty much obsolete they have alot of life as 90% of the cassete people have moved on to other formats. The most of them have not run as much as Beomaster or Beogram CD. Beograms get used until they give up. Beocords? Great sounding machines for little money.