By 1903 the gasoline engine was already recognized for its obvious benefits
An ad from the April 1903 issue of The American Thresher man for a Thompson-Lewis engine highlights the growing application of gas engines as agricultural beasts of burden.
From the same April 1903 issue, an ad for the Flour City tractor produced by Kinnard-Haines Co. Kinnard-Haines was an early entrant in the gas-powered tractor industry, producing its first tractor in 1900. Powered by a stationary engine adapted for traction duties, the tractor was rated at 20 HP.
April 1903 ad for Garllus & Spooner engine. Faintly visible to the right of the flywheel are the words "Chicago Engr. Co." It's assumed this was the actual manufacturer and that Garllus & Spooner was simply a jobber. No information has been found on either company, a not particularly surprising fact given the profusion of engine manufacturers that appeared around this time.
April 1903 ad for Columbus Machine Co. engine shows a nice cam stopper sideshaft, perhaps a 5 HP model. This engine also features Columbus' interesting evaporative cooling tower (visible just behind the flywheel), which utilized a series of pans, the cooling water dripping from one pan to the next.
April 1903 ad for Fuller & Johnson showing what appears to be a 1-1/2 HP pumping engine.