Stiff. Sometimes the runners are actually stiffer than they should be for easy steering. But as a rule the stiffness is traceable to the binding of the various moving metal parts. First examine the way the sled is put together, and as you move the handle back and forth, notice the points at which there is some give, or slippage, between wood and metal. Clean rust from metal at these points. Slip sandpaper between metal and wood and sand wood lightly. Rub paraffin on wood surfaces that you can reach, and squirt powdered graphite on inaccessible surfaces. Oil rivets holding metal to metal
Seat broken. The planks are screwed or nailed to the wood cross braces but may be riveted to the rear strap of the metal steering yoke. File off the rivet on the broken plank, then unscrew the rest of the plank from the frame. Replace with a new hardwood plank. If large-headed rivet like the original one is difficult to obtain, replace it with a nut and bolt. Be sure to use lock washer under nut.
Center plank attached to steering handle cracked at end. Replace plank if you wish, but it can be reinforced adequately by screwing one or two short steel mending plates across the break.