I can't speak for Revell in the 'States, but the U.S. branch of Revell in the UK were OK with me on a couple of occasions. I once converted their 1/32nd Phantom F4J into a RAF FG1, and needed the drop tanks, and other parts, that weren't in the kit. Rather than scratch-build them, I contacted Revell UK, asking if it was possible to obtain these parts, which I knew were in a different version of the kit. They sent me a complete sprue, with all of the parts needed, plus whatever else was on the sprue, free of charge!
Apart from that, I would think they'd have to offer replacement parts, as part of the 'fit for use/ marketable quality' laws, a version of which will be in place in the 'States. But, who knows? They might not be responsive I suppose!