You can build a 40-50cc conversion engine from a chainsaw for a LOT less than any of the commercially available engines.

Check out the conversions forum. There are lots of Craftsman and Poulan (same thing) chainsaws in this size and you can get them put together with electronic ignition for under $300.

I've done a number of these conversions. They are usually a few ounces heavier--sometimes not. They are ported and timed to run in the higher RPM range. So keep that in mind and prop them for 7500+RPM on the bench. Anything under 7000RPM and they are kinda doggy.

I have done 2 of the Poulan 54cc engines. I spent under $300 for each conversion. I bought the engine--had it machined--sent it to someone for a hub, sensor, and ignition system. Bought a BCM muffler and started flying it.

I've done a few of the Ryobi 31cc engines. Probably not enough power for your Decathlon--but fun and very easy to convert. Keep them UNDER 8000RPM unless you want to see a connecting rod detonate and fly apart.

If your worried about them being ugly--they aren't. You got a cowl on this plane--right? You'll never see the engine.

If your worried about performance--then I'll remind you that MOST gassers on the commercially available market are chainsaw/water pump conversions. DA, 3W, ZDZ are all specific RC built engines.

Brison, Zenoah, BME, Fox and numerous others are ALL nothing more than a factory converted industrial engine. They use the SAME cylinders, cranks, rods, pistons, carb...etc as a chainsaw engine. Usually the only thing different about the "professionally" converted engine is the case. Take a look at a BME 3.2 engine. The ONLY thing thats not originally from a chainsaw is the CNC machined aluminum case.

Don't be scared of a conversion engine. Save yourself a few hundred $$ and do a conversion.