A thicker gasket will correct the condition where the gears are snug and need a bit more play to operate. I have some notes in this thread that might help (note: conclusions in the first post are wrong)
http://www.gasgasrider.org/forum/showthread.php?t=9914
Check your gear mesh by taping up the assembly (as shown in the post). This should be the first step so you know the gear mesh is OK.
Rob, what does the bendix feel like when it is out of the bike? It has to move really smooth. If it is too dry the sprag clutch might be worn. You can see the sprag (one-way) elements when you slide the spring cover down. They allow for over-run when the engine starts. They have to hook up when cranking. It sounds like yours isn't throwing out at all (along the cam groove). Too dry to slide or maybe the cam groove is worn from being dry?
Any better operation using a car battery?