OK: So it doesn't appear to be a "seized" piston. I don't know if this motor has "Plain" crankshaft bearings...OR.. Ball bearings or Roller bearings. If it has ball or roller bearings..
Maybe..... this diagnosis
might be legit.
Perhaps ask the mechanic...specifically..WHAT.. he thinks the problem is.
Someone else might chime in here...I have a problem with the price quote.
NB