Question about xr 125cc dirt bike?

A 4 stroke 125 is fairly fast but not super crazy fast, bikes are faster than quads, there's less friction to be made up and weight. it'll be as fast as a 100-110 brand name 4 stroke. lifan makes good motors they are compairable to a honda just with more plastic on it. As for mph on the bike hard to say but it'll get up a go pretty good as long as you don't weight a whole lot. My nephew has a ttr 110 and i'm 6'3 180 pounds and that pull me around super fast so I wouldn't worry about power you should be fine.