If a player is regularly pushing on the flop with top pair, you should call him with an overpair, top pair and a good kicker, or anything better. With a flush draw and two overcards you have about 15 outs, giving you around a 54% edge ignoring any cards that improve their hand. It's a borderline case. It may be worth a call in case they're bluffing, and to stop players bluffing you out in future. Similarly a flush draw and straight draw has 15 outs, so it's also borderline. A flush draw with one overcard only gives you about 45% to make your hand. You should fold, unless there's a chunk of money already in the pot (say around half the size of the push).
Pre-flop with AK it depends what you put them on. Realize that you need to flop an A or a K to have anything, and you have around a 50% chance of hitting that on any of the five cards (actually it's 48.7%, so a shade under 50). AA or KK completely dominates you, but you're in with a shout against most other hands, so if their range includes any non-pairs I'd probably call as well. With AKs it would be a pretty easy call.