cap holding rod bearing came loss from crankshaft pin holding piston to rod connected to crankshaft came loss. you are better off replacing engine instead of repairing it.
Most likely there was a lubrication failure resulting in an increase in friction causing a catastrophic failure within the engine . The piston , the piston rings , the wrist-pin , or a bearing failure not receiving enough lubrication caused a piston to exit the block . Metal on metal = a thrown rod which is connected to the piston and the connecting rod is connected to both the piston through a wrist-pin and connecting rod connected to the crankshaft . See video tutorial below .
The piston rings may be worn out .
It would be the one closest to the front.
worn piston rings
my guess would be bad piston rings
With Coasting I presume the sound is coming from the engine yes? If that is the case then the engine may be out of timing. this is easily adjusted but i would still only recommend having this fixed by a qualified mechanic. more details on the timing: Every engine is carefully timed. the knocking may be created by the piston hitting the closing or opening inlet/outlet valves on the way back up. this can cause severe damage to the engine as the valves could potentially bore through the piston resulting in possible damage to the piston walls as it slowly starts grinding its way through the engine walls. you may want to have this seen to.
The engine would smoke and damage the bores.
If you only use one piston ring on 80cc bike engine, the compression ration will be reduced.
Which engine would help.
Excessive smoke from the exhaust, and an engine miss. If you remove a plug wire while the engine is at idle and the noise is gone or almost gone then that would indicate the cylinder that the piston is bad.
A bad ring would probably chatter in the cylinder and cause serious oil consumption. Knocking would most likely be either bearings (typically rod) or piston slap which is a common trait of a 350. Piston slap isn't serious as long as it clears up within a few seconds of starting the engine. Rod knocking means you get to rebuild/replace the engine or wait for a rod to go through the side.
Valve, valve spring, piston, piston ring, cylinder wall, head, headgasket, diluted oil, stuck injector,