The conversion of existing diesel engines to natural-gas (NG) spark-ignition (SI) operation would reduce U.S. dependence on oil imports and curtail greenhouse gas emissions. As the literature shows that the combustion process in such converted engines is different compared to that in conventional SI engines, understanding the effects of the diesel geometry and fuel effects on the in-cylinder flame propagation is important for optimizing engine operation. This paper describes the experimental setup that allowed the visualization of combustion phenomena inside a single-cylinder diesel engine converted to single-fuel NG spark-ignition operation through the addition of a spark plug and a low-pressure gas injector. The synchronization between the piston position and image acquisition was done using over-the-counter electronic components. While the setup could not visualize flame propagation inside the squish region, the combustion images, together with the pressure-based analysis, help understand the characteristics of lean NG flame propagation inside a diesel geometry, which is an important for designing a highly-efficient combustion process.