Like whunter said, the head looks unusable. The cracks between the prechamber and valve seat, and between the valve seats killed it. When looking for a used head, you need to use caution. Every used head on these engines I've seen tend to have cracks radiating from the prechamber, which is not bad as long as they are not over 1" long or so (I've used heads with those cracks without a problem). But you will need to make sure none reach the valve seats before buying.

To clean the block, you can use a 4-1/2" angle grinder with a fine wire cup brush instead of a grinding disc. Apply light pressure. Won't damage the block and helps speed up the cleaning process.