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I've seen dual 28BYJ-48 direct drive just recently https://www.thingiverse.com/thing:1597741 , but I'm convinced that even this will not work in long therm. The problem is in the side pressure to the motor shaft. It will require some bearings to support the hobbed drive gear as was in the original delta extruder https://www.thingiverse.com/thing:200078 .
Send me a PM with your mail and I will send you DWG file with required parts. But again be aware that any belt tensioning will put side pressure to the motor shaft which will result in motor jamming, so it is probably not good way to go unless you will add also a bearing on top of the motor pulley.