Hi Neil,
 
Sorry to be so late replying. The conversion is reasonably straight forward and is detailed in a Lyc SB (sorry, can't remember which one). The only way to do this is working from the front of the crank. I would strongly advise against drilling out the rear bung as it is almost impossible to make sure you get all the swarf out. Use a stout screw driver or chisel to bash a hole in the bung and lever it out. Don't bash too far through as there is a pipe running diametrically through the crank not too far behind the bung. You will then have to fit the front bung - requires a good whack with a 3lb hammer (with piece of wood in between).  
 
The alternative is to leave the rear bung in place, fit the front bung, and use the pipe that would have supplied oil to the prop to return oil to the sump - a special cover plate is required on the governor pad to let oil flow back.
 
Pete