A few potential sources...
1 - Check the Yahoo Inca group as there are frequently posts there for used parts and Inca gear. http://groups.yahoo.com/group/incawoodworking/
2 - Call Jesse at Eagle Tools 3027 Treadwell Street Los Angeles, CA 90062 (323) 999-2909. They are one of the sources for Inca parts left over from when Inca gear was sold in the US years ago.
3 - Inca is still in business. You can also reach out to one of their European distributors for parts. Given the small size of the gear, shipping should not be a lot. A website with phone number is http://www.doebeli.ch/v1.x/index.html
Be sure to click on the UK flag in the upper right corner for an English version.
Good luck - I have several Inca tools and they are still running strong. Please post what you find out.