I've "repaired" several parts during construction, including for kit parts that did not fit and drawings that were unclear as well as my own boo-boos. This was standard expected practice for early kits and there's nothing wrong with making a repair if it is done properly.
J stiffeners are cheap but the shipping will be the killer because of the length. So I believe it's worth ordering extra stiffeners, as well as angle stock, to be included in a shipment when ordering a kit.
As for this specific repair, I think a doubler would be fine and AC43.13 has enough guidance for how to do it.