Look for mouse nests in the airbox When I worked at a shop we would get sleds in that had nests in air box and the mouse piss would rust up the crank bearings. If it is just frozen on the top end try a some thin oil and turn it over using the clutch by hand
Hey, isn't that the stuff they sell as Sea Foam??
Have unstuck a hundred bike motors fresh out of the barn.
Here's whatchado.....
Mix up a jug/can/bucket of my secrete formula...stuff.
Use EXACTLY these amounts....
One gallon of diesel fuel, preferably no 2 with
One gallon of gasoline...cheapest stuff you can find....
One QUART of 3Ow oil...(or any old oil you have laying around the barn, even used motor oil will work)
Shake well, do NOT drink...even if you are thirsty
Now pull the plugs and use a funnel to pour some of the mix into the top of the motor. For bikes, I fill the crankcase too....right to the top...this lets everything soak in the agent.
Leave the plugs out and let it sit for a week.
Next weekend go out and grab the primary and work it back and forth slowly and steadily...don't force it. If it is still tight....add more juice and wait another week.
It has NEVER failed to unlock any motor yet. Just a time thing.....
Something about the combination of diesel and gas that will penetrate and loosen it up over time.
Try it...let me know how it works for yeah!!!
BTW I learned this from an old farmer/plumber...long gone, but he sure knew what he was doin....