1/4" x 20 for the governor hole, 7/16" x 14 or 20 for the low oil sensor.
Don't know about using a self tapper, you may end up with metal shavings in the crankcase. Most just pull the side cover, tap the holes, and then can clean up any metal shavings.
Something else to think about if you are going to run the engine hard. You may want to add an additional breather outlet for crankcase pressure.
I just tapped the governor hole for a fitting instead of a bolt, and then ran a hose connected to the stock breather hose up to a filter.