this design extends both motors out 9mm suiting the steel axles better
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I used the same motors as the original:
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https://www.banggood.com/Machifit-13GA050-6V-150rpm-DC-Electronic-Speed-Reduction-Gear-Motor-with-Metal-Gearbox-For-Elevator-p-1341680.html?p=5G2717320323201310AJ&utm_campaign=Adrian&utm_content=3312
(the link seems to have Adrian202's banggood affiliate :))
I used these steel axles:
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https://www.banggood.com/2PCS-Upgrade-Metal-Driving-Shaft-For-WPL-HengLong-RC-Car-Parts-Crawlers-Military-Truck-p-1267445.html?p=H5151494967462017032&custlinkid=895638
These axles might work as well: (I haven't tried them, but they look nice)
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https://www.banggood.com/Transmission-Drive-Shaft-6x6-For-WPL-B14-B24-B16-B36-JJRC-Q60-RC-Car-Parts-Silver-p-1535880.html?p=H5151494967462017032&custlinkid=895643