2013 Polaris Sportsman 500 HO no top end
 

Quad runs and starts etc fine. Can hit max RPMs. The top end speed is 30ish.

I have replaced springs and belt. Everything seems normal except no top end speed.

What I would do:
1) confirm good compression
2) after #1 checks out, pop the belt cover off, get the rear axel off the ground, start it, but it in gear and visually watch the primary/secondary clutches movement while on/off the gas...making sure both clutches are shifting out/in fully

Sounds like buttons gone in the secondary.

:yup: check the secondary clutch buttons unless it has a roller clutch. sounds like it is not shifting up and would be a secondary clutch issue