Problem with husqvarna wrk 125 engine

ENGINE 2 stroke single cylinder direct reed valve crankcase induction Liquid cooled 125 cc engine Cylinder size 124 8 cc Bore and stroke 54 x 54 5 mm 200 cc engine Cylinder size 175 3 cc Bore and stroke 64 x 54 5 mm 250 cc engine Cylinder size 247 7 cc Bore and stroke 72 5 x 60 mm 280 cc engine Cylinder size 272 2 cc Bore and stroke 76 x 60 mm 300 cc engine Cylinder size 294 1 cc Bore and stroke 79 x 60 mm I Cant put periods or commas so bare with me

when reinstalling the head you most likely damaged the timing chain or it's bound up. another possible issue is it's put together wrong, i.e. cam gear, timing chain guides. without the spark plug in that engine should roll over by hand... Changing valve size isn't a part of the problem. If you swapped a different cylinderhead to do this then is it part of the problem ???

Rock the bike gently forward and back while shifting into fifth gear. Now push the bike forward while sitting on it and see if the rear tire turning will turn the motor. If not, your motor is stuck. If it turns, you have a kickstarter issue.
Many people never learned to depress the kickstarter down until you feel it engage before kicking it through. Kicking from the top slams the gear teeth together and causes damage. You may need to pull of the engine cover to determine if there’s damage.

Apparently a gear from the kick start level has become jammed which is why the engine won't turn.

sounds like the gears inside are stripped or missing teeth open up the side case and check and replace any gears missing teeth or stripped

2 stroke engine oil for the engine goes in the oil tank under the seat.
motorcycle gear box oil goes in the gearbox, car oil may cause the clutch to slip.

It is a 2 stroke motor there is no oil you put in the motor only 2 stroke oil in the tank so it will mix with the gas.

You have a broken tooth on the first gear set. This means two gears will need replaced; the drive gear and the driven gear in first gear range. DON'T just replace the gear with the broken tooth. Replace the gear it meshes with also.