Hi!
How does the engine behave without cowl?
Having a cowl with entrance and larger exhaust opening isn't enough, you must duct the air so it goes true the finns on the engine! That is very important!
What exhaust system do yoy use? I guess it's small silencer as the rpm is soo low (6400rpm wih a 23x8 prop).
In comparance my MVVS .45cc (Evolution in the US) turns a 24x8 Mentz prop at 6500rpm using the MVVS small cannister silencer.

How does the front ball bearing feel like? Do you have much play in the bearing?