Genuine sluggish and will not go past 27mph.

[Swagman]

I have a friend with a rough house 50. It will not go past 27mph. It was at the local genuine dealer who apparently replace the carby, ran without the exhaust for testing. Replaced the cdi. They apparently replaced the gearbox oil and cleaned the reeds.
We have access to a running rh, so we replaced the car and air box as a set, cdi, exhaust, bariator, belt and clutch. Still sluggish and slow.
When we replaced the exhaust it made things worse. When we replaced the air box and carb it ran 20 max.
I have seen a few posts in the past with no resolution.

Any ideas? Especially since a trained dealer was unable to get a handle on this issue.