There isnt that many 125 pit bikes. Most are 110. However pretty much all of them are shite chinese copy engines.

If you want a decent engine get a honda xr (crf) 50 and then get the takegawa 110 kit for it. Or buy a Kawasaki klx 110 engine. Both will cost you quite a bit.

Most go kart engines (if they're not geared) are honda generator engines.