Is there a reference for the composition of viral genomes?
NCBI houses a collection of viral genomes at the aptly named Viral Genomes portal. They currently have 5921 viral genomes and by going to the previous link you can browse sequences from the major viral families.
If you have a nucleic acid or amino acid sequence it can be searched against NCBI's sequences here.
Uniprot also have a viral protein sequence annotation project which they describe as such;

The concept of this website is to associate specific knowledge for each virus family with viral protein and genomic sequences. All the available information is presented in a concise and accessible virus fact sheet, associated with the ordered list of reviewed entries. The site covers the whole known virosphere. The database section allows access to UniProtKB protein entries, as well as to specific virology databases.
Our efforts are currently directed towards viral ontology and reference strain annotation. A robust ontology will allow a more precise definition of the involvement of each protein in viral replication steps. The reference strain annotation will become the benchmark for each of the 334 genera.

Their work can be accessed through uniprot.org or through the project portal at; http://viralzone.expasy.org/