Trevor,
I have heard of a PVC valve fitted to a modified oil fill cap, with a hose then leading away to the rear of the subframe. I made a one way valve and fitted it to the inside of a modified timing chest cover on my Mark 3 Sports early on, with a new larger vent tube down to the road. I also know of a DT sprinter in the UK using a PVC valve with some success.
I have not had trouble with oil leaks at the magneto joint face (thought it seems a common complaint), so have not drilled a drain hole in the timing chest. There should be a hole already to allow the oil level to rise up just so the bottom train of gear teeth (camshafts) dip into the oil, before draining back into the crankcase proper. You should not go lower than this. The only leak on the engine I have is a slight dampness at the bottom joint edge of the rocker covers. And probably because I do not trust to tighten down on the screws really hard, as the covers had been cracked once before.
So I would try the PVC valve first before drilling drain holes. It may be all you need.
-Doug