So it,s ya transmission gears that,s f u cked, not ya kickstart shaft...????
Dont even try to replace gears with out re-newing crank case first, this may be what sheared teeth off in first play, (play).

, you,ll be just throwing good money after bad.
OOracing and Rhino-bloke, dont seem to have much bits for the Lifan150 engine(apart from a Gasket set)...dunno why.
Got loads for 140/125,s ..???
Keep an eye out on Ebay, somebody usually breaks a engine, with useable bits ya needing.
If ya stuck, try this bloke in Germany, he,s usually got a load of Lifan bits. (got a crank off him)
http://stores.ebay.co.uk/Happy-Motor-Parts-and-More