Not really the best at modifying rules, but here's my crack at it
1.) All Small Arms take a -1 on their to-hit rolls vs. a motorcycle (Does not effect Heavy Weapons)
2.) No attacking unit takes the +1 for firing at a "large target" like they would if they shot at an automobile.
3.) Motorcycle Damage Table
d10 Result Motorcycle1...........................Motor Hit
2...........................Motor Hit
3...........................Tire Hit
4...........................Tire Hit
5...........................Crew/Passenger Hit
6...........................Crew/Passenger Hit
7...........................Crew/Passenger Hit
8...........................Crew/Passenger Hit
9...........................Weapon Hit*
10.........................No Effect
*Only applies if the motorcycle has a sidecar attached. If no sidecar, then this result counts as "No Effect."
How does this seem? Any more thoughts?