Although I have no experience doing this, most auto parts stores will carry a rethreading kit. Helicoil and Time-Sert are some pretty well known brands that offer solutions. I suggest searching for "sparkplug rethreading" and learning what is available to help. Perhaps someone will have specific recommendations for you, but anything is better that what you have now. If it were me, I might start with just carefully running a tap into the port (after taking off the head) to try and clean up the threads before I consider putting in an insert.
For the gas cap gasket, Yamaha sells them. There are also aftermarket ones that I have seen at outlets like MikesXS and HVC Cycle in the US if you want to go that way. Yamaha part # is 2M0-24641-00-00.