Depends which one.

If it has a single throw crankshaft then you are trying to balance two pistons and rods with the counterweight, surely a single piston and rod of a similar capacity engine would be lighter.

Also the firing impulses are at 180 and then 540 instead of every 360 degrees. Still, they run fine.

Now if you are talking about the two throw crankshaft models then the pistons and rods balance each other, the slight rocking couple due to the cylinders being slightly off set can be mostly balanced by a counterweight, and they fire every 360. Beaughtiful.

Back to the OP.

The primary out of balance mode for a single cylinder engine is at engine speed and in the opposite direction. This is due to the counterweight not quite balancing the weight of the piston and rod at TDC and BDC, but being too heavy to balance the big end of the rod half way between.

To balance the primary out of balance moment would require a second shaft, rotating the opposite direction with a (smaller) counterweight beside, below or behind the main counterweight. Even then there would still be a secondary rocking moment.

All in all far too much trouble considering that our model engines run surprisingly smoothly all things considered.