Welcome to RCU.
Yes some of the 4 stroke engines can be converted to gas. CH Electronics has the stuff needed. Since gas engines run hotter than glow engines you may have a problem keeping the engine cool. You need to get more specific on the engine to get a better answer. Usually the larger engines are easier to convert such as 1.2 cu-in and up.
Ken
http://www.ch-ignitions.com/