Attempt to contact the author (likely to fail, since the module is sooooo old). Contact the PAUSE folks to request a transfer of ownership or maintainership. If it needs to be on CPAN now, upload an unauthorized version (bump the version number too). Once maintainership is granted, bump the version again and upload (so that it gets indexed).
Your OO module is a new beast entirely, and deserves its own name. Math::Brent::OO or Math::Brent::Class, possibly. That way you don't break an existing module (and unlikely as it may be, a dependency on that module) by converting it from functions to an OO interface. The email@example.com mailing list is a good place to vet module names. Be sure in your new module to document clearly how it differs from the old one; Simpler OO interface, more thorough test suite, portability as a design goal, etc.