Actually, you can do it. And way back when, before glow was invented, gas spark on .60 and smaller engines wasn't just the norm, it was the only way to go.

So, why did everyone go glow on the smaller engines?

Power.

Glow engines produce more power for less weight in a given displacement. I've even heard of some of the giant scale racers converting gas engines to glow operation for more power. And it's no accident that dragsters use fuel that is a lot closer to glow fuel than the stuff you get at the local gas-and-go.

You can take an off the shelf glow engine and covert it to spark and gas if you want. The only mods you'll have to make involve getting a spark, and, maybe, some carb changes. You will run cheaper, but you'll get less power.

If I recall correctly from what I've read, not having done this myself, glow engines burn fuel faster, so the carb openings need to be enlarged when going from gas to glow. But going from glow to gas, all you need to do is lean out the high and low engine needles most of the time. But the mid-range might not track quite as nicely as it should, since it's expecting a glow-type mixture. Probibly depends on the carb though.