It's tedious, but I'd honestly go through the trans, count all the teeth on each gear and figure out the total reduction ratio. Once you know that, set a reasonable top speed goal and use a gear calculator to find the Kv that'll get you there. Most toy grade cars don't have adjustable motor mounts so you're probably stuck using the same size pinion as on the old motor. So unless you find a way to make the mount adjustable you can't use gearing to adjust top speed, you'll have to use motor Kv.

I haven't seen the gears like you have, but I'm pretty sure a modern brushless motor will shred the gears of a radio shack toy. Going 2s is a smart choice, finding an esc with punch control would be wise as well.