I have WRC 1:8 pan car diffs in stock that are used in our current racing chassis that may work for your purposes. Our latest diff includes a machined aluminum pulley and I also stock belts and break shaft pulleys for the car. Feel free to contact me for more information.
You can view our latest thread here:
WRC Racing - 1:8 Pan Car Forum
Irwin RC 1/8th Pan Car Specialists - www.irwinrc.com
SPM Competition, WRC Racing, MIP, Novarossi