Well there's a great thread about reverse engineering the points costs for things either here or on BoardGameGeek, I can't recall which it is. That could be a great way to purchase abilities or stats to level up your pilots for an RPG style campaign. There's a tabletop game I enjoy very much called Hornet Leader that uses a lot of pilot management mechanics that handle the stress of sending pilots out on multiple sorties in a quick amount of time, you might want to peer through the rules of that and get an idea or two to make your X-Wings more heroic and to simulate not being able to have your ace pilots fly every mission due to stress.
Now onto the Star Destroyer… A small flight of X-Wings attacking a Star Destroyer head-on would be downright suicidal, so perhaps there's a specific objective that needs to be performed around it, or this particular Star Destroyer is heavily damaged. One feature I like in several other games that feature huge boss-fights is the ability to attack hard-points for different effects. Perhaps not only the turbolasers should be available targets, but also the shield generator, the communications array, the hangar, even the bridge or engines.
For turbolaser turrets, I would say they can only fire at range 3 or possibly 2 or 3. Any closer than that and the traverse speed of the turret would be too slow to track a starfighter. I would use some kind of mechanic where it's very difficult for a turbolaser to hit, but if it does, then it hits for immense damage. Perhaps when rolling to hit, only count critical hits, but have them deal 2 damage to shields or a standard critical hit against the hull. As for hull and shield points, that's a matter for playtesting probably. Your typical ISD has 6 Turbolaser turrets and 2 Ion Cannon turrets on top of the hull in two banks, each with a 180 degree firing arc, leaving a blind-spot directly in front of the nose and along the deck of the ship. There are a number of other cannons mounted along the sides of the ship, but for the sake of the scenario, you can assume your ships are staying high to keep out of the line of fire of those cannons.
The biggest threat for starfighters attacking an ISD is the number of fighters they carry, 72 TIEs to be exact. But for the scenario, you could call it a surprise attack, perhaps the TIEs could only be scrambled to defend so fast, effectively giving the scenario a time limit before TIEs pile up so thick that the players can't deal with them any longer. If you have the miniatures for it, I would say a good representation would be that every turn, roll an attack die, on a hit or critical hit, a flight of 3 Academy Pilots in TIE Fighters appear on the board.
Like I said earlier, it might be cool to allow the players to attack hard-points individually. Taking out the shield generator might remove the shields from all other hardpoints, attacking the bridge or communications array could reduce the flow of TIEs to represent the chaos the lack of leadership might create. It could also be a mission objective that must be completed in a certain number of turns so the Star Destroyer can't send for help. Likewise taking out the engines might be another objective to keep the enemy from escaping into hyperspace.