I've thought about it some more and if you could try and drill some really small holes through the arm. You'd need to be really careful about hitting the wires inside. And then get some really small gauge steel wire like 30g or smaller and loop it through and around the prop motor and super glue at the break. Yes this will add some weight to that side, but I don't think any more than epoxy resin. You really should try some really good super glue first, but I'm not sure if it would hold. Epoxy works pretty good, but I've had problems with it failing as well. When I've used epoxy resin or super glue on things like eyeglasses or other plastic pieces that get any kind of strain placed on them , they always eventually broke. I learned if I use wire as well it usually works much better. Unfortunately anything you try besides super glue will add weight. A foot (0.3 meters or so) or so of really thin steel wire will probably weigh less than a gram. Enough epoxy resin to encase the broken arm in a few mm of thickness will easily weigh at least a gram. This is the kind of stuff I'm talking about as an example. https://www.amazon.com/Artistic-Wire-Beadalon-Stainless-30-Yard/dp/B005ZUVBLC/ref=mp_s_a_1_5?crid=34FRCB03U49YU&keywords=30g+steel+wire&qid=1637083619&sprefix=30g+stee%2Caps%2C171&sr=8-5