167 mph sounds about right. :laugh: I think you may be neglecting to factor in the primary drive ratio (crankshaft to transmission via clutch) of 3.722:1. To complicate things a bit more, I'm pretty sure 4th gear isn't 1:1, but fairly close, 0.958:1, I think, a slight overdrive. So, you have to factor the primary drive ratio, the transmission ratio, and the final drive ratio.
Lifan Horizontal Engine | Lifan Thailand
FYI: There is another Lifan 125 (1P154FMI) with a shorter stroke and larger bore (54mm x 54mm) that makes 11.4 hp @ 9,500 rpm (vs 7.8 hp @ 7,500 rpm of the 1P52FMI); it usually costs slightly more, $314 + $60 shipping was the first deal that came up in a search, that includes everything except an air filter and exhaust.
I see someone beat me because I took too long checking some info, but I'll post anyway.
Maybe a 17 tooth countershaft sprocket would get you where you want to be, top speed wise. If you don't already have one, a tachometer would be a wise investment since you are wringing it out.
Here is some countershaft info, but I would measure before ordering a sprocket.
https://tboltusa.com/store/tech.php?pID=168/Front+Sprockets+or+Countershaft+sprockets
According to that info, this sprocket
should fit; TBolt is good about replying to messages, so you may want to contact them.
https://tboltusa.com/store/large-front-sprockets-17t-420-p-1951.html
They also have a 16 tooth sprocket.
https://tboltusa.com/store/large-front-sprockets-16t-420-p-1952.html