You can usually move your cog out 1mm by using a thin bottom bracket spacer. Also, you might need different cranks.

Is your chainring mounted on the inside or outside of your crank? Most converted road bikes will need their chainrings on the inboard side of the crank. It doesn't look as clean, but it's what most of us have to do.

Also, some chainrings have the teeth just a little off center. By flipping the chainring around, you move it in a millimeter or a fraction thereof.

Are you using what was formerly a triple crankset? On a triple, the middle ring will be further out than the inner ring on a double. Usually too far out. If that's the case, get a double crank and put it on.

I've found that 95% of the time, the inner ring on a road double lines up almost perfectly with the cog on a 120mm track hub.