Not sure we have had the same issues but I'm from MI as well so I'm dealing with the same weather (for the most part). The first time my engine was up and running I had a lot of spatter out the front of the engine. The mount and down tube were wet with fuel. I should say this was the second time I had the engine on the bike. First time it wouldn't start at all. Broken rings and the area where the head and cylinder met was shaped like a potato chip. I had the cylinder decked but it sputtered fuel and bogged on modest hills despite my 44t sprocket. I had bought the engine on CL thinking I'd save a few bucks. So then, I've replaced the cylinder with one purchased from BGF, rings also from BGF. Replaced the CNS carb with a NT carb and also the cable, and a new NGK plug all from DAX. Also bought a plenum on eBay from Turret Tooling (mostly for the low end performance). Now the only leak I need to seal up is the pipes on my SBP expansion chamber.
All in all the engine has been apart at least three times and the sputtering is nearly gone. The four stroking is getting to be less and less as the new parts run in. I took a shotgun approach to solving my problem. perhaps a $20 - $30 carb would do it instead of an entire engine for you.
Something else to consider - a 9 hp motorized bicycle is a motorcycle in MI, not a Moped.