When I first started making an electrical power assist for my Trooper, I started with the drive wheel from a power wheels toy for kids and 2 relays, full power. That worked fairly well for a while, much better than no assist, then Lenny said, make the power out proportional to the torque in. I rejected that idea for a while, but became absorbed with the idea and I have made many models and designs and trials. I'm very happy with the controller I'm using right now and it can be built by someone with electronics knowledge. It is set to be very sensitive so it needs less input torque than most, it also has the remnants of an earlier trial set up that didn't get removed, so I added a switch that allows me to choose between sensitive and not so sensitive. The power unit is an older 170watt unit from http://www.superatv.com/UNIVERSAL-Kits-C1039.aspx, but has been up powered with a motor from Banebots, which can handle more power than I can safely feed to it with out burning up parts.
So if you have a power unit and no controller, maybe you can build one or have some one build one for you. I have attached the schematic, a spice file is available if you contact me, this program won't allow me to upload it