Wrong way around Monkieh!
Go to a race, check what classes you got in your area, then choose one to compete in.
200cc four stroke? well then the GX200 is the engine for you...
BSP class? you might need a sealed block, which can only be bough from the race officials
(well a validly sealed one that is)
100cc 2 stroke classes? YOu need a 100cc engine..
125cc shifter you need such engine of course.. and
it MUST NOT be a motorcycle engine but a kart TAG engine
(engines like that cost around 4000 € new, so be prepared)
AND:
ALL OF THESE need different chassis.
a Shifter kart chassis is different from a 100cc DD chassis,
and both of them are different from 4 stroke chassis.
So yeah, in order to make a good decision if you want to spend time and money on competetive racing,
buy a kart you are actually allowed to race.
(there's no point in building a kart that you are not allowed to race, right?)
And there's no point in buying a crg 2stroke chassis, when the class you want to compete in ends up being a 200cc four stroke class.
'sid