Hi Alec,
Not sure what car you have but for LG45 the ribbed drums are pretty thin to start with and I would not skim them at all. Skimming would increase the likelihood of overheating and distortion and they tend to vibrate anyway which is why many have a piece of asbestos rope around the middle rib secured with a large jubilee clip over the top
The Club should know the original diameter as they have had new ones made and you can get new "Alfin" drums in cast aluminium with a cast iron liner from "Typecast limited. All expensive but worth it if you are going to drive the car in a spirited fashion.
If the scoring is not too deep new linings will soon bed in with use and the drums might be ok as they are but you would need experience to make that judgment. If you re-use the old drums de-glaze them, without trying to remove the scoring...
Hope that helps
Bill
