[Dave at Pelican Parts]

You can tap the holes without splitting the case, but there is going to be some risk of metal getting into the oil passages. This can lead to Very Bad Things... You can reduce your risk in a number of ways, but there will still be some risk.

You can JB Weld over the holes, or find your FLAPS and start looking through all their Loctite stuff (and similar chemicals) for a freeze plug sealant. I don't know the product number offhand.

--DD