Ok, well I would just go for a modified version of it. Buy a cake board and don't worry about the tracks unless you want it to be perfect, after all at 1 he really won't even know what tracks are.
Stick your sponge on the board and cover it in white ready to roll icing. Do the same with the smaller cake. Set the smaller cake down on top of the big one. The first thing you're going to do after blowing out the candle/s is dismantle the cake so you can cut it, so don't worry about attaching the small cake to the big cake, just carry it carefully.
Get some writing icing for the birthday message on the side, and roll out some ready to roll coloured icing, or colour your own using food colouring (you don't need much, and the more you add the sloppier it makes the icing, so have spare icing sugar on hand and knead everything well. Roll it out and cut out the playdough car shapes)
Stick them to the side of your cake using a teeny bit of apricot jam and a pastry brush (brush on the car shapes not the cake) then add detailing using writing icing and sweets for headlights etc. Just use two or three colours for the cars, he really will think it's awesome even if it isn't!
As for the massive number 1, I would cut it out of card and add some glitter, then sellotape it to two cocktail sticks and jam it into the top cake. After all, you just need it to last for the big reveal and a couple of photos, it doesn't have to be edible!