You can just use the file recipes/hama.properties
The options that you care are the following:
#
# Customise instance types & AMI for Amazon EC2
#
#
# Use a specific instance type. See http://aws.amazon.com/ec2/instance-types/
whirr.hardware-id=c1.xlarge
#
# Ubuntu 10.04 LTS Lucid. See http://cloud.ubuntu.com/ami/
whirr.image-id=us-east-1/ami-35de095c
#
# You can also specify the spot instance price: http://aws.amazon.com/ec2/spot-instances/
# whirr.aws-ec2-spot-price=0.15
Also, if make sure your setup your credentials file ~/.whirr/credentials
and add these lines to it.
PROVIDER=aws-ec2
IDENTITY=<YOUR AWS ACCESS KEY ID>
CREDENTIAL=<YOUR AWS SECRET ACCESS KEY>