Once you reach the heads-up of a Sit & Go, it's time to shift strategy again. During most of a Sit & Go you are keeping ICM considerations and payout jumps clearly in mind. Once you reach heads-up, you are simply playing for whatever is left between first and second, so any other considerations go out the window. For this reason you need to re-adjust and simply consider your chips to have a linear value, then play based on that.
If you suspect an all-in call would be a 50/50 and you already have chips committed then you should now automatically want to take that gamble. This is very different from three-handed where with other players remaining, doing so might be a bad idea since they benefit from it.
Fight or flight
When you get heads up, you will typically find that either the blinds are very high and that the effective stacks are quite short (below 15 big blinds) meaning you should usually adopt an all-in or fold strategy. If you are deeper (above 15 big blinds) then there is room for you to make small raises, limp and play post flop. But most times it will be push or fold.
Playing all-in or fold is quite simple and is essentially a solved game in that optimal play charts can be found in books such as The Mathematics of Poker, or Secrets of Sit & Go’s. These charts give you maximum shoving or calling values for all hands.
However it is worth remembering that most players play too tight in such scenarios and that you can move all-in with a lot of hands from the small blind for 10 big blinds such as 97o, or J3s. Similarly if your opponent is very tight then you can move in with more hands than these tables recommend. But remember that if players are shoving with a smaller range of hands than is optimal, you need to call with less hands to compensate for this.
Going deeper
With deeper stacks there is little point moving all-in and risking all your chips when standard raises to around three big blinds will often do the same job. If you are called you can still afford to continuation bet, giving yourself two chances to win. The most important thing about playing heads-up when deeper is adjusting to your opponent and coming up with a winning strategy.
For example if they are too tight you can raise most buttons, but if they are aggressive you may need to tighten up but be prepared to call all-in with some marginal hands like small pairs, weak aces and high cards.