I would stear him towards the Maxon A-Max model, comes in various sizes and voltages, I have used the 12mm and 16mm models. Your builders choice will depend on the motor housing on the machine but they have diameters from 12mm to 23mm in the that model..

Just a side note, some of the Maxon models are too powerful (IMO) for a tattoo machine... They still work but you lose fine tuning of the machine via voltage from the power supply so I would recommend getting a Motor that runs 7-9v rather than the 12v model you suggested...

The 16mm A-Max runs in that voltage range and would be a good choice if it will fit in the machine...