If you're building a toy like a go-kart sort of thing then a BLDC motor will probably be your best bet, they are available for RC models, some quite large, up to 10HP or so. If you want to convert an actual car to electric then forget it, 20V will never cut it, there's a reason real EVs use several hundred Volts. You need several tens of kilowatts to propel a fullsized car at a reasonable speed, that's a lot of power any way you slice it and if your voltage is low that means the current has to be crazy high.