So this is a chassis and extra details that I designed for the Porsche 356 chassis that UWcharlie1983 published (https://www.thingiverse.com/thing:3017039). It can be printed without supports and the parts painted and glued together.
This was made with Tinkercad and it did need to fettle my 2nd version a bit (eg. cutting the rear axle slot so the bushes would pop in, filing the front a little so it would clip inside the chassis).
The guide also sat a tad too high so I needed to add small washers and file down the top of the guide housing. It now has constant contact with the tracks.
I did make the necessary alterations at the design stage, so this is a better (untested) 3rd version I've uploaded for you. Of course more changes can can be made if others want to continue with this design.
I stuck two 20x5mm magnets (one above and one below) the chassis in the middle position and it works well.