I assume that the motors run on 12V?
if that's correct just get a couple of 12v batteries,
my advice is don't go to a scrap yard, the saving isn't substantial enough for what you're doing, and the quality is so low that you'll probably never get a full charge out of the batteries anyway.
as Ronco said, if you have car batteries, use a car battery charger, it is possible to make one, but ideally just go buy one, it's safer.
you said that you wern't using a petrol engine because it's too expensive.
but now you're looking to buy 2 car betteries, (£30 each) and a charger (£30) so you're spending ~£90 (or whatever the US equivellant is...
by the time you've spent £90 on batteries and chargers you could have gone to a scrap yard or a private sale and bought an old moped with a 50cc engine, or even a 100cc or a 125 cc engine, you'll have more power that you can shake several sticks at.
the first scrap bike I bought was an MOT failure, I bought it for £20 the engine worked perfectly (it was a 125cc) so there was plenty of power.
plus it came with everything needed, in the way of serious braking gear drive chains and all.
if you're looking for serious power, go with a real engine.
if you're looking to do it on a budget then go with a real engine.
besides which, once you've gotten old with it, you'll be able to sell a petrol kart much easier than a electric one.
just my 2c,
(it's obviously a lot easier to make an electric kart though)