To scratch-build or kit-bash a slot car, there needs to be a body, a chassis and the necessary running gear. That's the case for any scale of slot car and whether the aim is to race the car competitively or not.
The body can be crafted from scratch in many different ways, such as carving wood (as shown below), plasticard construction, or using moulds. Alternatively it can be adapted from a plastic kit or bought off-the-shelf as a resin, fibreglass or lexan shell. A body usually requires painting and detailing to produce a slot car model that is unique and personal to the builder. Decals for thousands of cars are available to buy or can be designed and printed at home.
More info for Pendle Models. 2.1 mi 96-106 Every Street, Nelson, BB9 7LS. Pendle Slot Racing. Pendle Slot Racing A plethora of models every shape and size! Penelope Pitlane Classic open wheelers at their best. George Turner models What can I say, the master of all things resin! Kits for assembly T & T Hobbies A nice cross section. RMS Resins Some classic North American Muscle FPF Models Some nice pre ad open wheelers amongst these.
Sometimes a chassis is supplied with a kit (eg George Turner's kits), but there are also many adjustable 'universal' chassis that can be used (eg Pendle's PCS32, Penelope Pitlane, Slot.it HRS2, MRRC Sebring, numerous metal racing chassis etc etc). Some builders will find a chassis from another slot car model that will fit or can be adapted. And there is always the option to make your own chassis from scratch - using metal, fibreglass board, plastic sheet or 3D printing.
There's an enormous variety of running gear available to scratch builders and kit-bashers, although most of the adjustable chassis can be bought with all the necessary components included.