The piston & rings may or may not be toast. The big end WILL be totally shot, and the PTO main bearing (n the sump pan) will probably be beyond salvation. You will need more or less the complete set of internal parts, and even then it will probably smoke heavily because the replacement piston & rings will not have the same wear pattern as the cylinder.
A new engine is the only sensible solution, but you need an auxilliary PTO engine, and they haven't made them for years.......
Best compormise - examine the lower main bearing when you get the sump off. If it's OK, find a standard (non aux PTO) engine of the same type. Swap the sump pan (complete with PTO components) and camshaft (with the drive gear) from your existing engine. Take note of any shim washers.
phil_saunders(a)bigfoot.com
Phil Saunders, November 2009