Is there a more robust transmission I can put/fit to my Deere E180. I have a lot of hills to mow and I pull heavy stuff. I have rebuilt the trans twice and made a significant increase in the trans oil viscosity. All efforts have improved the operation for a while but mow time gets less and less with increased oil temperature. The stock trams just will not perform. Are there heaver duty tractors whose trans can be fitted to an E180?