Please keep it simple, regarding what I need Thanks

[Saracen]

My son and I are both Autistic, there is no way I can afford new electric bikes.

I have a good Saracen Mantra from 13 years back and my Sanderson life frame cruiser I built.

Both are 26, 3bys, and I would like to add a drive to them that is UK legal.

I have NO idea where to start other than here.

We ride off road, gentle, but that is not easy at 65.

ANY advice will be much appreciated

Thank you