Methods to improve the tropism or other features of a virus are disclosed. Such methods can be used to prepare, e.g., DNA or plasmid libraries of variants of a gene encoding a viral capsid or envelope protein having a randomly inserted restriction site, libraries of viral clones with such variant genes with a randomly inserted restriction site or polypeptide sequence targeting a receptor expressed by a specific type of mammalian cells. Described are also methods to prepare mosaic viruses, i.e., viral particles wherein copies of one or more capsid or envelope proteins originate from different sources. These methods can be used to prepare mosaic viruses of a specific mixture of wild-type and mutant proteins, or of different types of mutant proteins.