They typically have a geared counter balancer mounted right next to the crankweb on the engine's crankshaft.
The problem with out model airplanes is having a prop and spinner on the front. every time you change the prop out , the positions is a little different. Plus the weights of the prop and spinner change too. so it would be most difficult to keep it in balance.
Then there is the nature of a single cylinder engine too. Everything is against you achieving a perfect balance.