That might be a sticky exhaust valve, which can be caused by overheating. Is the carb and timing set correctly, and are all of the cooling fins, flywheel screen, and area under the flywheel shroud clean? I'd check all of that stuff and start using Marvel Mystery Oil in the gas.
A 109 is not 'underpowered' in my opinion. That 'bogging down' you describe may be caused by either improper carb settings or timing, as well.
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