www.w116.org/ should be a help.
YFM 350SE Big Bear 4x49725x8x1225x10x12
If you go to the Yamaha web site and register with the site, you can download a PDF of the original manual free of charge. This applies to many of the makes and models from Yamaha. The manual will detail specifics of the plugs needed as well as details on how to replace.