Motorworks do some aftermarket seats as a direct replacement to the originals, if your seat base is knackered. My mate had one on an R75/6 and it was nice. It also had a plastic base, so should not rust. Just not sure if they did an R65 version.
Alternatively, I have also replaced covers, both on an R100 and K100. Its a satisfying job, if you have the right tools - pop rivet gun etc, all the right bits - all in good condition, and the time.
Cheers
Rev. Light