If all you did different was add a clutch and chain,,, then I would check the clutch install first. If your clutch isn't turning freely the chain is stopping the shaft.
*Pull the spark plug wire for safety * and then turn the clutch ( real wheel off the ground) , see if it spins. Then pinch the clutch and see if you can move the shaft to see if the engine is froze.
Always test a new mod with the rear wheel of the ground and bike supported good.