Summary

These are a few adaptors for electric skateboard pulleys!

designed for the Backfire cloudwheel pulleys, but they work on any pulley

The 44T adaptor is perfect for 120mm cloudwheels. In the backfire remote, select "105" as the wheel size, in order to keep the precise speedometer.

A few considerations:

download and print the shim, you're gonna need it. Otherwise, the adaptor will rub on the plastic protector. Place it before anything else, between the shaft and the pulley.
The STL has 0mm clearance. yes, 0! it will feel snug to put on, and that's on purpose - it's not gonna fall off the pulley while you're cruising at 25mph. For the same reason, it's gonna be quite hard to remove. print it in PETG as it's more springy than PLA, allowing it to grip better on the pulley.
Make sure your elephant foot compensation is dialed in! you don't want it to not insert completely!
I have tested it and it works wonders for me, but regardless USE IT AT YOUR OWN RISK!
it's PETG so it's gonna wear down faster than normal pulley material, it still shouldn't be a huge problem as it just takes a few hours to reprint and replace it. If you have better materials, feel free to test and tell us in the comments!
if you want to try it, you have to print it slow, hot, with many perimeters and with an infill of ~65%, as it was tested to be stronger than 100%.
you're gonna need bigger belts for this. With the 44T adaptor, HTD280 belts are perfect, even with the non-adjustable mounts. I'm talking about the backfire Zealot, your mileage may vary with other boards.

How to keep the speedometer relatively accurate (44T pulley, backfire Zealot):

If using the 120mm wheels with the adapter you have to set the 105 option on the remote. It will over-read by 4.1% which isn't too much, every 15mph it will read 0.6mph too high.
If using 105 wheels, select 85 in the remote. It will under-read by 3.8% (at 15mph it will read 14.4mph)
If using 95 wheels, keep the 85 setting in the remote! It will over-read by 6,4% (at 15mph it will read 15.96mph)

Print Settings

Printer Brand:

Prusa

                                            Printer:                      
I3 MK3S

Rafts:

                                            Supports:                      
No

Resolution:

0.2

                                            Infill:                      
>60%, <80%

Filament: ERYONE PETG
white

                    
                        Notes:                     
Make sure your elephant foot compensation is dialed in! you don't want it to not insert completely!
if you want to try it, you have to print it slow, hot, with many perimeters and with an infill of ~65%, as it was tested to be stronger than 100%.