For those using a 3D printed chassis, here are some suggestions.
First of all, the printed chassis needed will depend on what car you have. For both the Fly Ferrari GTO and the Monogram Jaguar I used Shapeways/Olifer 3D printed chassis. Shapeways has a number of companies that they print chassis for, such as CG Slotcars and Slot.it 3D Print Shop. The Olifer chassis are the only ones I have. Here is a page of their products: https://www.shapeways.com/shops/3d_olifer?li=pb
Note that Olifer makes two different Ferrari GTO chassis, one which will accept either a sidewinder or inline motor pod and one which is listed as IL Pod. The inline pod chassis does not have the wider bump out at the rear and will require that bump out be cut off a Slot.it pod. The advantage is that the chassis is narrower, giving more room for tires and wheels. The regular chassis will require hubless wheels to fit under the GTO body. The Jaguar only comes with the narrow chassis. Olifer also makes printed inline pods that have already been narrowed. These motor pods have an adjustable rear axle height using the stock Slot.it spherical bushings. If the axle height is set to standard, inline with the motor shaft, regular crown gears will work. If the axle height is raised, offset crowns will be needed.
The front axle on the Olifer chassis will also use the Slot.it spherical bushings ( SICH56B, or robbed from another motor pod).
I like these adjustable axle setups.
For axles, any regular 3/32 axle will work. I am not sure of the length, maybe 48 or 51mm. I grabbed axles from my parts box. The ones I had were too long so I cut them.
For wheels and tires there are lots of options. One of the easiest, and the ones I used, were the Classic Wire Spoke Wheel and Tire sets from BRM, part number S106S. This includes 15 X 7 front wheels and 15 X 8 rear wheels with tires and the great looking wire wheel inserts. You can use these tires, after truing, or you could change the rear tires. Although it is not marked, I believe the rear tires in the set are 30 Shore. I used Slot.it tires on the rear, some F22s and some F15. BRM also has tires in those hardness. If you get BRM wheels make sure to get the wheels for 3/32 axles; they also make wheels for Ninco 2.5mm wheels.
For my original GTO with the GT40 chassis, there was not enough room for the hubbed BRM rear wheels, so I had to use CB Design hubless wheels. The only way to get the wire wheel inserts out of the BRM wheels was to carefully cut the BRM wheels. This also required shaving the inside of the CB Design wheels to get the insert to fit. With the narrow Olifer chassis, the BRM Wheels can be used as is.
You also have the option of using any other wheels and tires. You could use Slot,it wheels with whatever insert you wanted. The Slot.it Zero Grip front tires are pretty wide and may not fit. For my Jaguar, I created a rubber mold of the Monogram plastic wheels to create correct Jaguar cast resin inserts. These required some tweaking and I may not be able to create duplicates at reasonable cost.
You can use any inline motors that you want. I tried orange end bell, 25K motors and Flat-Six motors. They all worked reasonably well.