When entirely satisfied internals are cleaned out, check gear selection via input shaft and rear wheel rotation.
Gear selection ok
and clutch accessed as ok, reassemble with new gaskets. If gaskets unavailable, learn how to make your own. Fill oil, prime system with plugs out ign killed.
Clutch In Adjustment. Start engine and select gears...
Alternatively,
run engine till at normal running temperature with some 2t oil added to the fuel, engine cooled by electric fan.
Using the clutch handlebar lever, you may hear the clutch free off, noise change when clutch in and out. You may hear rotating shaft slow down.
Stop engine.
Bike on centre stand with front wheel up against a wall or similar, so bike CANNOT roll off centre stand.
Chain and rear wheel safe to be rotated at speed ?
Start engine and idle . Pull in clutch lever and get assistant to rotate rear wheel as fast as possible safely in correct direction. Drop into gear.
Clutch will not disengage, so blip throttle...
Oil heat, transmission snatch and clutch pulled in will encourage clutch to free in a gentle way.
A higher gear would be better but try first gear first. The clutch will free off eventually, this method works for me, but strip/clean etc. Is ideal method.
This still leaves clutch debris in the oil as well as oil cleaning/sludge lifting action of the oil.....
See exploded views, see what's involved, what parts are available ( valve covers, gaskets, clutch steel and friction plates etc.. ), decide action plan......
It's your bike, your desicion....
Dd23🤓