Stihl, Husqvarna, Tanaka, or Shindaiwa should get you a good running engine to convert in most cases but are just as hard to find cheap as Echo... The Poulan/Craftsman/Weedeater engines (are all made by Poulan) are usually super easy to find because they are the most disposable build but they are also the lightest of the consumer models. At one point I set up one of the old 18cc Poulan 4 bolt backplate engines with prop set up for gas/glow and it was right at 2lb. The downside of these is their out of the box power is quite low, their mufflers are quite restrictive (although some are super simple to gut) and exhaust port timing is VERY VERY tame, I think I measured one at under 120* one time... Open up the muffler and increase the exhaust duration and they might be acceptable, probably not on par with a stock Echo but usable...

Was flying with a guy on Sunday that had a Goldberg stick type plane (W8ye had one, don't remember what it was called) with a 2 bolt backplate 25cc Poulan on EI with the stock 2 needle carb and a stock muffler with 3 quarter inch holes drilled in it, sounded like a flying weedeater but once warmed up ran ok, got to messing around (he was told the engine was smaller, so I brought out a known 25cc to attempt to jump it up a little) and decided to gut the muffler at the field and it then sounded like a flying chain saw and according to him was a completely different plane in the air, looked to be fairly respectable power. I left him with the spare engine with the guidance of telling him to get the exhaust duration to 150*, we'll see how it flies after he plays a little .