The good news is that this is not all that hard of a problem.
You're going to want to look in three places.
First you'll want to make sure that the engine is staying exactly where you put it. If it's leaning to the left or moving in any other way, then you'll have chain troubles. Check that your motor mounts are up to the job. And if they're not, then you're going to want to fab up something of your own. But the engine can not be allowed to move.
Then check your drive sprocket. It needs to be flat and centered on that wheel.
If those two things check out all right, then you'll need to check the chain itself. I've never gotten a bad one. But I understand that they sometimes come out of the package in bad shape. Twisted links and such.
Best of luck to you.