Standard on those was a PC22 Keihin. There is a hard to find '24' from an XL175 or the carb ftom an ATC200. Have look on Ebay; there are any number of sellers flogging 24/26 mm four stroke carbies for as lttle as 30 bucks. Check that any choice has adjuster screws on the correct side. If you want it to go but are not too wrried about idle quality a oval slide PWK or OKO 24 or 26 will give good power.