I would agree that nothing here is proprietary or overstepping ones bounds, the fans are a weak point for many controllers and finding users a viable solution at a reasonable cost that they can do themselves is kind of what this forum is meant for...
That being said, my issues would have little to do with the RPMs, I would imagine that as long as it is spinning in excess of 1000 RPM or so, the controller will say it's OK, but the replacement should have a similar CFM airflow rating, if it is too low replacing the fan is irrelevant and does no good, if it is too high it should be fine as long as it doesn't draw too much current.