Gary what’s your budget? A CR125 stock honda “kit” might be the best way to go for what you are planning. Like Chris said taking a motor off a bike can be a can of worms. A “good” motoX motor can come to pieces pretty quick when you put it under the stresses of a kart leaving your with time and money down the tubes.
If it’s for fun, hell skip the CR125 and just go to a CR250, it wont be much (if at all) faster than a CR125 but it will be TONS of fun and the damn thing will lug forever. Shark Shifter have all the parts you might need for either of these projects.