Hi,

Is there play from side to side? Is there a "grumbling" type noise when the motor is running? With the plug removed does the motor turn over smoothly without any noise or catching? If the answer to any of these is yes then it is a good idea to replace the bearings. It is a fairly simple matter to replace the bearings. Remove the barrel and piston then heat the crankcase, wear heavy leather gloves, and tap the crankshaft, (with the nut in place to ensure the thread isn't damaged) on a piece of hardwood to remove the rear bearing. Reheat and use a wooden drift, (piece of hardwood dowel) to remove the front bearing.
Take the bearings to your local bearing service and they will match them. Remove the seals that face into the motor before re-installing.

Cheers,