Might be too late if you went ahead with purchasing a new engine....but just incase...
I had the same problem with my 05 TBA and pinned the sprag shaft in place. If I remember correctly, drilled about 1/4" to 3/8" into both the crank case and engine cover, and ran a pin through the center of the shaft. I don't think I spent more than $50 - mainly on the pin. Run about 10k miles since, no issues.
