So the question is: can anyone suggest some rules to adapt from vehicle-to-vehicle combat (or ship to ship combat) that I could use to adjudicate such things?
Vehicle Movement and Combat.
Edit - Don't be intimidated by the number of links either. In a nut shell, vehicle speed is split into four ranks (human running, 35mph, 35~80mph, & 80+). Like flight maneuverability, each rank increases the number of squares it takes to turn forty five degrees and each round you can adjust your speed category by one. Like the Ride Skill in D&D, the Drive Skill can perform some extra stuff as part of your Move Action, like a DC 15 check to dash or brake hard can alter your speed category or a sharp turn check halves the squares needed to turn.
Trying to ram stuff uses a die (d2/d4/d8/d12) based on your speed vategory and a number of dice based on your Size (med 4, large 8, huge 12, gar 16, colo 20), further modified by your target's direction of movement (side-swiped x0.25, normal x1, head on x2) but a DC 15 Reflex Halves, damage is applied to both vehicles and everyone inside of them, and you need to make a DC 15 Drive Check or lose control. Which is a fancy way of saying it randomly spins into going one of eight directions and if you failed by 10 or more the vehicle rolls, damaging it's self and everyone inside takes a bunch of damage.
Driving fast also improves your AC but penalizes your attack rolls, in addition each type of vehicle provides a specific rating of Cover (using the old 3.0 rules) but since you're driving an island I don't think you need to worry about Cover. For the most part, if you can handle anything less than Perfect Maneuverability flight on the table top, you can handle driving pretty easily.