[Max Lobovsky]

rather than using both sprockets and gears for the reverse system like green suggested, you could just use two different sized gears so that the smaller one has room for a middle gear while the larger one just meshes directly with another larger output gear.

small gear----------------------large gear
intermediate small gear [A]
small gear ---shifting dog-------large gear

note that [A] will be occupied by the large gears also because they are, well, larger than the small gears

and as for plastic gears, they won't in reality save weight because the weight of a plastic gear for the power it can convey is actually higher (unless maybe of some high performance stuff that you probably wont find a gear made out of) than the weight-to-power ratio for steel gears. I'm pretty sure even brass gears (i think most small non-plastic gears are brass) will have a better ratio. [edit] i realized how dumb and not scientific my "strength to weight ratio" sounds. I'm not sure which property of the material is the "strength" in this case, but basically, by strength i mean its resistance to shear which is probably the primary method of breaking a gear, shearing off its teeth. [/edit]