I have a Troy Bilt Bronco, vertical tine tiller. The unit looks very new & runs great. However, the transmission shaft & worm gears are badly worn for some reason. I could replace the shaft & gears but the cost is prohibitive for the value of the machine. I would like to find a good used transmission assembly if possible. The transmission model is 618-07010 (replaced with 918-07010) and the S/N is 1C115P20252. The tiller model # is 21A-70M8766. Any help is appreciated.