Historically Indians seiged rather than assaulted forts and the gaming would focus on breakout or sallying out as well as some foraging expeditions. Any Old West rules would handle this, LoToW or Yellow Ribbon for example depending on how many figures you wanted to use.
For a more agressive attack akin to Rorke's Drift I would use the Sword and the Flame with house rules for fires as indians frequently used fire as a primary weapon when they could.