clutch bell should spin freely and be connected to rear wheel via chain/gear.intergral with gearbox. if its the box i think it is, you should see a 12-13mm nut inside the bell. that isnt the issue. or you wont achieve anything by undoing the nut at this point.
you should be able to remove the box from the engine by undoing 4 bolts.check with a screwdriver by levering them, that the clutch shoes will move. if so, the clutch is fine.
now the issue is the box.
spinning the clutchbell should spin the output sprocket at 3:1 redux. or similar. but it will be a solid connection. once, again, if its the box i think it is. if nothing happens, its dead. if you saved your money and bought one good kit instead of two or now it will be three cheap kits... this wouldnt happen. (or 4 or five as you said elsewhere)
split the box itself open, and...either its the chaintype and the bearings/bearing bosses have failed, or its the geartype...same deal. a gears busted a key, a tooth, sheared a shaft... somethings broken. the cheap gearboxes always break and sound rough when they do work.
if its a chain type redux...get the gearbox from a 49cc mini dirtbike. the ratio is the same, the box is the same, but theyre made a lot stronger than the ones i see on the 4stroke kits.
the splined output shaft and finding a suitable sprocket is tricky. i have to bore a HT sprocket, and turn down an old splined sprocket and weld them together... or change the rear sprocket and chain to suit the finer pitch chain. #35 on some, #25 on others...
if its a gear drive type...save your money and get a real gearbox this time. search threads. the best types are named regularly.