Although I have little experience of Aero-engines, they have a great many similarities to 1930's car engines.....Are your compressions good? Camshaft timing correct? (1-tooth out can have an enormous effect) Ignition timing?
Mixture?...Prop has already been mentioned....it's a compromise,like a single-geared pedal cycle. even daft things like fairing strips to flaps, can tweak performance markedly.
Your engine may fail one or more of the checks above,you may think about an exchange engine if it's cheaper than rectifying the present one.
read the POH properly and try to contact other owners....it may be that you have bought a "slug" but wanted a "racehorse"
Friend had an Aeronca Chief...lovely stable forgiving aircraft and dead simple....but you could not be in a hurry and had to watch out if there were liable to be strong headwinds.