Just about any engine will convert but I like the ease of Ryobi conversions and lots of conversion parts available. 31cc is as small as I would go with Ryobi. Echos. These are real sweet especially the 23. cc. Lots of tricks you can use on them. Homelight seems to be fairly popular. I have one the has been very reliable. Poulan has decent power to weight ratio. Hear some good things about MAC.

Bigger the better if you can afford the plane to fly it on. I personally like the echos.