If you are going to buy one then get it quickly as they are selling fast.
There's no need to change the guide or the motor. As far as the motor goes I have standard 18000rpm Mabuchi Scalex and Fly motors than are in faster cars than those with upwards of 20Krpm SlotIt motors. Without the magnet it'll travel faster than with it (until it gets to the corners). Motors do not alone make a car faster as this is also relational to tyres, track layout and controllers, and other factors that vary from car-to-car.
But change the tyres as Scalex tyres lack grip. However, Fly tyres will not fit the hubs as the central spine is a different width. You'll have to either glue them on, or fit wire gap-fillers. Some people use tiny DIY water supply grommets that are used in household taps and garden hosepipe systems. If your repalcement tyres are not snug on your hubs then you'll get trashy performance.
Alternatively why not leave the car as it is, remove the magnet and replace the lost downforce with lead weight?
But get the car if you like it before it's sold out and the Ebay ripoff merchants start selling at silly prices.
Or, if SlotIt are going to make one, why not save a few $$$ and wait for that as it'll be a much better performer than the Scalextric version based on their previous releases.