Machine Mart do a 4 piece Stud extractor set which appears to work without need for drilling. Order Code is040210057. Alternatively they do "Irwin Bolt Grip" sets if you still have a head on yours (again no drilling).
I have a levelling plate on my RV8 - try
www.v8tuner.co.uk or Real Steel. I ended up using stainless steel manifold bolts from one of the above.