Hi Bill,
It sounds like you got a good engine. A larger or bored out carb will in conjunction with a unrestricted muffler give good performance. This will allow you to use more pitch or prop diameter. The ideal cylinder head temp. (CHT) is usually between 180 and 220 degrees F. The charge flowing past the transfer ports will also cool this area. I am sure that of the many used engines that you have disassembled you found evidence of the cooling effect of the incoming charge on the piston. I do think that the rpm should be kept below on the Ryobi because of the stamped rod. The piston is heavy and has thick walls which doesn't help. If you do a web search on Ryobi you will find a site called teaching a weedwacker to fly. I covers boring a stock carb to 400 thousands (about 13/32"). The 11mm walbros seem to work as well also.

Dave