Being that aircraft need lift to be useful, I would find a balance between weight, size and piston/cylinder condition. Likely, the first consideration would be weight, second condition and third size. Of course it would have to be your ultimate decision, but thats how I'd go about it. If none of these fit the bill, it never hurts to go to your local lawnmower repair shop. They always have scrappers lying around that still have a good engine in them. I have a shop I go to who will have at least 15 running weedwhackers in the basement they will sell for 10 to 20 dollars. In the end, the last thing you want is to be frustrated with your airplane's flight charactaristics all because you were too excited to get it into the air.[:@] Take your time and find what you want. A bit of shopping around will be worth it!
Good luck!