Getting a single cylinder engine to run without (much) vibration is not easy – and as you approach perfection, extra parts need to be added. Balancing bits that just go round is easiest – flywheels etc just need to have their centres of gravity in the middle of the axle. Anything that goes back-&-forth requires something else which goes forth-&-back in the same time, and on the same axis. It can be done, but books have been written about it. Incomplete books …
Cheers, Tim