As an owner of an open source project, I love hearing from potential contributors. Since you're interested in contributing, I'm guessing you also have some issue with the current software, either a bug you'd like to fix or a feature you'd like to add. Open an issue and then in a comment on the issue, express interest and willingness to do the work. If the owner is still interested in the project, I'm sure you'll get a positive response.
However, if the owner isn't interested (you get no response), check the license and see if you're allowed to modify the source and distribute derivative works. You can always fork the project and implement it anyway, for yourself and the community.