Do you mean an 87 Samurai
The thing that makes an engine sound
like a Diesel is bad loose pistons and
or worn loose Con Rod/Main bearings,
but you say you had the engine rebuilt,
so I would say you need to find out what
they did, and what parts were replaced.
Are the valves all in adjustment ? They
can be clattery if they are out.
Loose cam sprocket could rattle around,
I just rebuilt a 1.3 Sammy and the sprocket
was loose and had a divot in the slot where the
Cam pin engages it, this will change the cam
timing and could cause a loss of power, Also
aside from mechanical problems, check the
timing 10* BTDC is normal, too advanced will
cause pinging, and too retarded will cause a loss
of power and punch you described
Hope this helps, return and report
what you find, Good Luck
Wild
