Mike, if there had been overheating, there would have been a smell from the grass on the cylinder head fins and it would have been dark brown, verging on black, getting darker as you got closer to the surface of the fins.
If you start to dismantle the engine, take a look at the suction pipe for the oil pump to see if it is blocked or nearly blocked by sludge. You might also check the condition of the filter. Usually there is an automatic bypass for the oil filter if it blocks up, but the bypass can block up too if there is sludge involved.
Unless you are going to replace the whole engine, it is important to find out why it broke, so you can be sure you've fixed it and it won't happen again.
The pulley and extension shaft has been fabricated from three pieces of steel. The damaged pulley could be parted off and a replacement pulley welded on, if you can get a steel pulley of the right diameter and belt width. Provided the bore that fits over the crankshaft is undamaged, this may not be a very expensive job.
