[deejay]

The hammer noise sounds like a ballpeen hammer tapping the bottom of the tank, or even a marble bouncing around in there.

Check your tappets and rockers.

[Foggy_Auggie]

To me it sounds like a connecting rod big end bearing.

First get a Factory Service Manual from the host here.  Read it thoroughly and exhaust all possibilities before taking anything apart.  This post is just my own opinion and how I would perform it.  No responsibility for anyone else proceeding the same.

Get some help and remove the gas tank.
Remove the spark plug.
Remove the carburetor and exhaust pipe.
Remove the cylinder head and cast iron cylinder.
Check the wrist pin fit to piston by seeing if the piston can be rocked side to side in plane to the wrist pin.
Check for up and down movement of the connecting rod with the crankshaft at top dead center.  Gauge by feel on the connecting rod itself.  It will be possible to rock the connecting rod sideways slightly.  Factory up and down clearance is .003.  If DEFINITE up and down play is felt - it needs replaced.
If it is the connecting rod big end bearing - the engine is already partially disassembled to be removed from the frame.

Good luck.

[Jeri Danger]

Hi
Trying to diagnose a failure by literal transcription is just so damn
subjective, I would have said it was a valve train failure.
If one were standing next to it you would know instantly if it
was  big end or valve train or whatever
Brian, maybe you could post a audio file?Mpeg, etc?
Does anybody think that would help?
I am
Jeri Danger!!!

[McNasty]

I pulled over and shut everything down as soon as I found a spare inch of pavement so I could walk back and pick up my airbox. Then I waited about 15 minutes before I kick started it up and gently rode it back home, (remembering the shifter's quirks this time around).

- Brian

[Foggy_Auggie]

Hi
Trying to diagnose a failure by literal transcription is just so damn
subjective, I would have said it was a valve train failure.
If one were standing next to it you would know instantly if it
was  big end or valve train or whatever
Brian, maybe you could post a audio file?Mpeg, etc?
Does anybody think that would help?
I am
Jeri Danger!!!

You are quite correct Jeri!