Hi. My name is Ken Wilson (Starfighter).
Have converted both a ST 2500, and a OS BGX to gas. Both used a Walbro carb and CH ignitions. Both engines run FINE, w/o the problems alluded to by others. Any engine will overheat if not properly cooled, and the gas conversion does not increase the potential for this occurrence. (The ST 2500 turns an 18 x 8 prop @7850rpm using gas. Sold the BGX.)
* You DO NOT need to convert the engine to a roller bearing. You do have to run 10% oil with the gas because the engines do not have roller bearings, but no problem, and NO problems with any carbon bulid up. Use a high quality oil.
* You do not need to use a Walbro carb with the ST 4500, or the OS BGX as the stock carbs will meter gas with proper adjustment w/o problem. If using stock carb, get the Cline pump/regulator, or a Perry to ensure consistent fuel pressure as you cannot use muffler pressure when using gas.
The cost is reasonable for the ignitions, and the results are satisfying.
Another suggestion: The ST on ignition using FAI fuel (80% Methanol, 20% oil) is a MONSTER. Truly a BEAST in power output, even better than running gas/ignition. This option combines the reliability of ignition, with the use of a familiar fuel.
This is my opinion, and it is based on years of experience (modeler for 50yrs., flying R/C for 40yrs.). I rarely respond to these posts, and only when I can provide correct information.