Vibration when running?

Vibration is BAD. As Pol said, it puts unnecessary stress on the airframe. It can even shake any nuts, bolts, screws, etc, loose, or even off.
No single-cylinder reciprocating engine can be perfectly balanced, but the manufacturer gets it as close as possible.
What kind of balancer have you used on the prop and spinner?
Is the hole n the prop true, or slightly cockeyed?
Same for the spinner.
Does the point of the spinner wobble about visibly when the engine is running? (I don't mean is it loose!)

And buy yourself a stepped reamer! Then you can't go wrong!

Are you running the engine with or without the spinner? If with, take the spinner off and try it. It is possible that having the spinner on, with the previous prop, has distorted it, so that although it is static balanced, it is not rotating balanced.

All single-cylinder engines have some vibration, so it could also be that your fuselage is fairly flexible, and making the visible vibration worse as it waggles about.
What you could also try is running the engine with a smaller prop, to see if it is torque pulses that are making the tail end wag, rather than off-balance vibration. By putting a smaller prop on, you reduce the twisting load on the fuselage each time the piston fires the mixture in the cylinder.