William,
You can drill and tap the CB sump plate and it's been done successfully however, some have tried and failed thus causing them to buy a new sump plate. A portion of the CB plate needs to be milled perfectly flat to remove part of the cooling fins and provide room for the drain plug and mating surface. Once that's done, then you drill and tap a hole in relatively thin material that doesn't provide much material for the pipe threads. Where "SOME" have failed (not everyone) is that the aluminum plate is thin and it has cracked.
Another problem is that once it's drilled and tapped, it may not seal properly unless you can find the right kind of drain plug with a sealing washer.
For those of you that have been successful in drilling and tapping, I'm not saying that EVERYONE will have problems or that EVERY CB sump cover will fail, just that "IF" it does crack or won't seal then you're stuck buying a new plate
Last week, Bob Kelly and I spoke extensively on this subject. I've installed several of the Mainly by Design sump plates and they are made well and seal properly. My suggestion was to shell out the money and buy the manufactured plate and be done with it. Sometimes the work involved to modify something isn't worth the expense of the completed product that you are able to purchase.
Some people that have been successful will fault what I'm proposing but after many years of 'tinkering' with these cars, VW engines, and related options, I've found that """sometimes,""" it's just easier to buy the product rather than modify it.