Ypu can change the port definition in the target group for the port definition you're looking for.
The port in the target group is the port on which the targets receive traffic. The port on the listener is the port on which the load balancer listens for requests.
So you should set port 80 on the listener and port 8001 on the target group.
You may have to recreate the target group for that. I don't see a way to modify the target port without recreating it.