Hmm, there's a whole lot to unpack here:
1. The internal drive on the tachometer has to be there unless the tach case is empty. Does the gauge look normal from the front?
2. The drive is pretty easy to see: it's a small square within the larger mount where the cable goes. It should be visible with the naked eye.
3. The same square drive in the gauge is also present at the engine, with the square usually sticking out a bit.
I don't think I understand when you say the square is too small... the cable is too short or the size itself is too small? Could be that your tach drive (the square piece) is rounded out?