Linux kernel source used in Amazon provided Ubuntu AMI?

Amazon does not provide the Ubuntu image. The best Ubuntu image to use is provided by Canonical (makers of Ubuntu). The following link provides information about their image and verifying that you are using the Canonical Official image.

https://ubuntu.com/server/docs/cloud-images/amazon-ec2?_ga=2.260988877.1510486085.1690809525-1388295542.1690809525

Ownership verification
Users can verify that an AMI was published by Canonical by ensuring the OwnerId field of an image is 099720109477 (in the default partition). For the GovCloud partition, the OwnerId field is 513442679011. For the China partition, the OwnerId field is 837727238323. This ID is stored in SSM and is discoverable by running:
aws ssm get-parameters --names /aws/service/canonical/meta/publisher-id
With the value returned by that command, users can then run the describe-images command against an AMI ID and verify the OwnerId field matches the above ID:
aws ec2 describe-images --image-ids $AMI_ID
Note that listings on the AWS Marketplace will always show the OwnerId as Amazon (e.g. 679593333241). In these cases, users can verify the Amazon ID and look for aws-marketplace/ubuntu in the ImageLocation field.

The following link provides some information on using Custom Kernels on EC2 and how to get started from an existing image that is close to what you need. https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/UserProvidedKernels.html