Youve probably got a few spare hubmotors...so drill out a belt pulley for the 6-hole disc brake mount on one, then cut up an old bike frame for tubing/dropouts to weld up into the mounting bracket to put the hubmotor in place of the old AC motor, aligning the belt pulleys. The pulley will probably be a much larger size than the one used on the old motor, cuz the hubmotor will probably spin slower than the old motor, so less need to gear it down for the fan speed you want.
Or get more ambitious, and switch from belt to chain (more efficient), and use the sprockets and bike chain off the old bike frame to switch out from the pulley/belt. You just have to figure out the right gearing for the motors speed at the voltage itll be running at on your solar setup.
(Id considered something like this for my evap, back before the fire when I still had one, but never got around to it cuz i also wouldve had to build a new casing cuz it was rusted out anyway).
I dunno about the solar part; youd have to poke around all the solar threads for that stuff.