I have had several engines that had flat cams over the years and you can get by with just changing the cam & lifters. However, you really need to know why the cam went flat. If it was for lack of oil, there is going to be more wrong with the engine than just a cam & lifter set. The cam gets oil from oil passages in the brg journal, the lifter bore and the run off from the heads. The oil passages that supply the cam with oil also supply other parts of the engine and if the cam was starved of oil, so were the other parts that are down stream from the cam, so there will be other damage.
I have also seen cams go flat simply because it wasn't broke in correctly or it was a used cam and they didn't match the lifters up with the lobe it was originally matched to.