Can anyone help identify this engine

Generally unless you dress the teeth down on the hole saw you loose about a tenth of an inch in the cut. A 2" hole saw makes a 1.9" roller and so on. If you will give me the target max speed you are shooting for I will run it the calc at say 4K rpms and determine the size for you.

I would suggest a 2.5(2.4 sawn). That a good across the board roller. Will cruise 22mph@3000rpm, around 25@3600 and top just under 30@4000. This is keeping the engine in a comfortable rpm range, would probably top 5K rpm(37mph) if you make it. With 3hp it should do hills fine. If not drop a hole saw size or two down. A small reduction/increase makes quite a difference.