Used scooter lacks power.

[SunSpine]

Hello. I did a few searches and couldn't find anything exactly like what is happening with my scooter. I was hoping someone could help.

I bought a used scooter. It is marked a Venture ZN150T-7C. The engine is labeled BN157QMJ. It's a 2008 with about 2,400 miles on it.
My issue is that this scooter should go about 65 mph, but I can't seem to get it above 35 in an ideal flat situation. This scooter only goes about 18 mph hour up hills which is a problem for the commute I intend to use it for.
I replaced the engine and gear oil and that is when the top speed went up to 35. Now I need to know what the next step is.
Thanks.