Electric karts are not the norm, so setting one up is going to be hit or miss. I suspect it's going to be pretty heavy, with the batteries hanging anywhere you can mount them. So scaling the thing might be a waste of time.
That said, get it as close as you came corner weight wise. Say 45% front weight, 55% rear. (or whatever the TopKart info suggests) If possible, get the RF and LF within 5 or 10 lbs. The RR and LR will be whatever. Keep in mind a TopKart was never designed for an electric motor and a bunch of heavy batteries.
Also keep in mind old tires will not help your efforts. New tires will cure a bunch of ills.
Set the toe straight up and don't worry about it. That's the least of your issues. The kart will likely be so out of balance, 1/16 toe in or out is not going to make any difference.
Gearing is anyone's guess. Find out the motors likely RPM and gear it to run about 50 MPH. That will be a little math project for the students too. RPM / gear ration X tire circumference, blah, blah, blah.
Good luck, make sure the kids have fun, keep us informed, and post some pictures.
Clark Gaynor Sr.