what you have there is a enduro kart ...
knows as a laydown, those are only run on big tracks--road racing.
the ephassis is on straight line speed / low drag, not corner speeds. it still will be fast in the corners though. Keep in mind on large tracks the corners are not as sharp as a regular kart track. ... Although at speed they will seem sharp ... lol
with a 125cc laydown shifter your looking @ 125-130 with a good set up.
most like that are run with 100cc no gear box, air cooled., some TAG engines, not sure how many 250's you will find on that type chassis.
this is a laydown racing, they run body work nowadays
here is a 125 shifter kart, sprint type ... note: front one has kart based engine and rear one is running a bike engine. These are the type I have