Tough to beat a Tilly for ease of tuning.
CVK GY6 carb should be Keihin round jets, clones available around $3/ea, cheaper in multi-packs.
http://www.ebay.com/itm/Carb-Main-Je...efba2d&vxp=mtr
110 was the main in the original application, prolly a decent start.
http://www.mrcycles.com/oemparts/a/h...7ad/carburetor
You'll likely have to shim the needle .5-1mm and/or spin a little more taper in to the last 15mm or so, or swap for a thinner one.