Update:
I waited all day by the mailbox for the FedEx truck, got my motor, put it on in the dark. Without a load it was fine, but as soon as I put a small load on it, it had weak power and smoke started coming out. Not a good motor I guess.
Putting the sprocket on the D shaped shaft worked out fine, though!
It looks like I'll be taking the good motor apart tomorrow, to see if I can mess with it myself and not have to send it off to the guy. I don't want to be without any motor.
For those who are worried about my cabling, my cables did overheat when the motor overheated, and I want to replace them. My friend here where I'm staying gave me some. They are unmarked, but fatter than my slightly toasted 6-gauge.
I'm going to figure out how to post a picture of the cut end, though, because it is a silvery color. I'm wondering if they might be that copper coated aluminum stuff.
