Engine identification

[Claes.Lagercrantz@eu.pnu.com (Claes Lagercrantz)]

  Hallo,
  
  I have a Spitfire 4 (Mk1), 1963, with commission number FC5008L. The 
  engine is not original. It has a singel Stromberg carburettor and the 
  engine number is: RD8468E.
  
  Are there anyone out there who could give me some data about the 
  engine (compared to the original).
  
  year:   ?
  cc:     ?
  bhp:
  torque:   ?
  advantage over original:   ?
  disadvantage over original:   ?
  
  
  Best Regards,
  
  Claes Lagercrantz
  Spitfire '63