RE: Which engines to look for?
In the weedwhackers and small blowers, the Echo, Stihl, Ryobi, and Homelite all make good conversions. A lot of the chainsaws are labor intensive as the engine is cast as part of the whole saw. Some of the Echo, Homelite, and Poulan saws have the engine that is easily removed from the rest of the saw.
When possible, you can look up a parts diagram of the machine in question to see how it is put together. This works well for all but the Stihl engines, as Stihl only publishes it's mechanical tech info for licensed dealers, though the Stihl engines themselves are excellent.
The small "Weedeater" and Poulan brand weed whackers don't make particularly good conversions. They are light, but low on power unless extensively modified.
AV8TOR