You can cut a 45mm hole 90mm back from the bellhousing forward face with a hole cutter, according to some Morgan compensator modification instructions. It's not too tricky as it's fairly soft alloy and was there to allow greasing of the old style compensator. Phil Bleazey cuts a larger hole in the same sort of location to allow replacement of the Centa rubbers with the engine in situ. The hole size and location isn't critical, although be aware of what is behind it, don't drill into the Centa Unit.
I don't think that plugging the hole in the bellhousing is a cure for the leak, it will just move it or mask it. The person who built the car would probably agree.
