Heat the engine with a hair dryer (heat gun, etc).= before trying to start it. Get it nice and hot! This will expand the cylinder/sleeve and free up the piston.

If this engine is new or not broken in properly, you have to run it at light to medium throttle few times for 5 minutes at a time, let it cool and then repeat. Then drive carefully for a few tanks, getting it tuned and add speed.

This is typical of new/tight engines. Once broken in, the engine will be firm, but the piston will move more easily.