Ebay is your best bet for a crank or complete motor. You will probably have to buy the complete bottom end, I just picked one up for 160 bucks shipped. There were no aftermarket cranks made for the 85-86 motors. There are a few of them on ebay right now. There is another forum that I was informed about and it is www.everything2stroke.com, they have a little bit of stuff over there. I know there is a freshly rebuilt motor in the For Sale/Wanted section. He wants 500 for the motor plus shipping.
Jeremy Smith
2006 YFZ 450 50th Anniversary Yellow
Curtis Sparks Full Exhaust
K&N Filter