Rivets in vintage tube gear were a popular cost cutting measure, but if you ever find yourself needing to replace whatever they are holding down, you'll have huge regrets. I have also worked on vintage riveted amps where the rivets were slightly loose and causing lots of problems, and the only way to get things working again was to drill them out and replace them with screws.
In the SEX amp specifically, we provide hardware to insulate certain terminals, and that won't be compatible with rivets either.