Has the bolster collapsed or is it just a hole? Several of us have replaced the covers which is not too difficult (I managed it!). A photo of your seat may help track down a cover (cloth, leather?) if that's what you need. Alternatively try MGTF seats on eBay to see what's available second hand.
Richard
1.8i Mk2 Solar Red, 16 inch square spoke wheels, MGFMania hood with zip-in glass rear window, DRLs, Kmaps ECU, Pipercross panel air filter, MGOC Supersports back box & some cockpit bling