Drilling and tapping cast iron is no particular problem.
You will actually be drilling out the screws which will be a (most likely) mild steel.
A sharp bit used at a slow speed with oil will be fine.
The goal is not to let the bit get to hot or you will lose the edge on it.
When tapping use tapping fluid or penetrating oil, turn the tap in until it get tough to turn, back it out to clear the chips and turn it in again.
Repeat going forward and back with the tap until you are done.
I would use the cover that came with it and seal it with gasket material (at auto parts stores) or silicone.
It would be good to buy some brass or stainless steel screws to put the cover on with so this will not happen again.
At the least put some "Anti seize" compound on steel screws
