The tyres on the Porsche are standard Scalextric large slick tyres. Any slot car dealer can supply them. If you contact Scale Models or Pendle Slot Racing or Scalextric Restorations or other dealers they can send you some. These tyres were widely used by Scalextric in the 70's, 80's and early 90's on Porsches, Rovers, BMW 3.0 CSL, Triumph TR7s and loads of old F1 cars.
If you google them, you will also find Ortmann tyres that will fit and give more grip then standard replacement tyres.
Hope this is OK
Matthew