What rubber plug will fit this hole?

I believe this hole was intended for the HT-lead on older Vespas, but is plugged on motors with external HT-coil.

My old rubber plug is now too mushy, so I need to get a new one, but which will fit this ?

The hole is 12 mm in diameter and the casting is 5 mm thick.

See picture on: http://www.sonicdesign.se/vespa/Vespa_rubber_plug.jpg

It's a 1962 GL motor.