I think you can put two 9V batteries inside the enclosure of the MX-100. They use to last for at least 10 hours. You can also use some rechargeable ones and use the existing DIN-connector for an external charger.
There are a lot of things you can change in the setup of 3x Nakamichi microphones and the MX-100. You can supply the microphones from the mixer if you like. You will need to bypass the output transformers inside the microphones, replace the 2-way jacks on the MX-100 with 3-way ones. The third wire supplies the microphones. You can also replace the heavy enclosure of the MX-100 with a lighter and smaller one (but that still can take a pair of batteries inside). As the MX-100 have no balanced inputs, use short cables to the microphones. If you have a recorder/soundcard with balanced inputs, you can modify the MX-100 to balanced outputs.
Roger