I don't think any other seat, bar 500/126 would be a direct swap. But loads of people on here have put in different seats. I am using Cobra classics in my 500 (126dave had these in his 126), and I know a few people are using various bucket seats. The only problem with fitting them is that you will have to modify the original runners, to make an adapter or new subframe. I think the best person on here for this is Mick (bis13) as he made an excellent subframe out of an old set of 126 seats. If I remember rightly, he posted up some instructions on how he did it. Have a look through the search and you should find it
Dom