50cc topping at 25, low compression?

[qwertydude]

When was the last time a valve adjustment was done?

It's entirely possible you have low compression too. Combine this with worn CVT components and you could easily get lowered top speed.

At 10k miles if the compression is low I would actually just replace the whole engine. You may be in for a costly rebuild. In order to return it to like new you would probably need to replace the rear pulley because those can wear out causing poor shifting, new roller weights, new CVT face, new belt. Also at 10k miles you'd likely have to split the case and replace the connecting rod because if you put a BBK it could lead to lower end problems on a well worn engine because as it is on a new engine it's tough to get 10K miles with a BBK. I'd wager putting in a BBK your engine would go no more than 5K miles after the rebuild.

New BBK is the cheaper more reliable route. About $375 from Scrappy's. So it's 100% complete and new so you'll get more reliable miles rather than trying to rebuild an already worn out engine with worn out transmission components on top of that.