I just dissasembled the gearbox on my kids mini mini buggy. I used a FNR gearbox from a lawnmower on a fully caged gokart frame with 21" atv tires. It is a Spicer gearbox. It amazed me how simple it is. The input shaft is a pinion shaft. 90 degree from it is the output shaft. On each side of the pinion shaft is a ring gear. In between the ring gears is a coupler that is keyed to the shaft. It has notches that engage the ring gears for forward and reverse. I think I recall this is the same setup used on outboard motors. The gearbox is packed in grease. It is about to give up, when it does, I will clean it up and take pictures. Tim