Hi Mike,
The gas engine forum really would be the better place for this post and you'd probably get a lot more responses there.
I'll try to answer some questions since I made the switch to gas this year.

Conversion engines are just that - they were originally meant to power a weedeater or something similar but have been converted over to be an airplane engine. Some, from what I've heard are quite good but I've never flown one and don't have first hand experience. They usually tend to be cheaper as well than a DA or 3W engine. Desert Aircraft, 3W, BME, ZDZ and others are built specifically for model airplanes. They have a wide range of sizes also from 20cc Zenoah to 300cc Evo from Fogues Research. Generally, conversion engines tend to be in the 26cc - 40cc range *I believe* but don't take my word for it on that.

Ignitions - since gasoline engines use a traditional spark plug, they need something to make an electrical current to fire the spark. Glow plugs "stay hot" due to chemical reactions with the nitro, etc. and don't need that. Spark plugs can get their power generally 2 ways.
[ul][*] Electronic Ignitions - a battery pack, switch and ignition module is required. The module draws from the battery pack to fire the spark plug. The timing is usually controlled by the ignition module. [*] Magneto Ignitions - a spinning magnet, generally on the flywheel generates the electrical current. Magneto engines tend to be harder to start and are heavier due to the magnet but less complex since you don't have a battery pack or ignition module.
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Most gasoline engines use a Walbro Carb so a fuel pump isn't necessary (the carb pumps). Gas engines usually require less tuning due to temperature and humidity and usually are fairly forgiving even when running a little rich.

Gas engines also vibrate quite a bit - especially the single cylinder gas engines. So, when you go gas - buy loc-tite and use it on EVERY BOLT. Constantly do maintenance checks as well.

The best thing you can do is just talk to people who are flying gas. If no one in your area flies gas, visit some IMAC competitions and have a look at their planes, talk to the pilots and get a general understanding of the setup.

Oh, don't start your engine alone!! Always have a helper. I hope some of this made sense and best of luck to you!