Back in the old days all small engines were gas (or deisel ). All you need is an ignition system.

In the old days ignitions were pretty simple: a set of points driven by an eccentric on the crankshaft, a sparkplug, and a coil and condenser.

You could still set one up with points but it would be better to use a magnet and hall-sensor. For a sparkplug use a champion V2 or equivalent which will screw right in the place of your glowplug.

You'll still need to run about a 20% oil ratio so don't expect that this kind of conversion will be clean like the larger purpose built gassers. Also you'll need to use gasoline compatible fuel lines, etc.

Here's an example of two of my old time gas powered engines from the 1940s. On the left is an Ohlson & Rice .19 sideport, and on the right is an OK .29 Mohawk which is more like a modern day engine in design. You can see the points or "timer" assembly on the fronts of the engines just behind the prop flange. Both of these engines run fine on gasoline.

Back then when the glow plug was invented most guys couldn't wait to convert their engines from gas to glow, so they stripped off the timers and replaced the spark plugs with glo plugs and threw away the ignition coil and it's batteries.