Oil Drain plugs differ. There is no such thing as a common plug. You are going to have to get it out anyway, so once you do, you can measure it and find a replacement. It is going to be a metric plug as the transmission is an Italian product. The strange thing is, if you look at the parts book, they call it a 1/4" G plug W/magnet. And the crush washer is called a copper washer 1/4" 13X19X1.5. This makes no sense at all, sounds like a 1/4" pipe plug but it isn't. The washer dimensions give you a hint as 13mm is about a 1/2" so a metric drain plug (that uses a crush washer) of about 12mm diameter with a straight thread is most likely what's in there. A little antiseeze on the threads when you put it in helps and watch the torque. People tend to overtorque them for fear of them falling out.