The DLE 20 will be a good power choice for the CG Extra 300 plus it beam mounts similar to a glow engine but will sit a bit farther out on it's mounts due to the rear carb arrangement. I use a TNT L/gear on my Extra which works great and like Gary L Snr I used a tapered 3/8" to 0" hardwood wedge to give it a more froward rake to help stop it's nosing over tendency when flying from grass fields, plus it gives sufficient clearance for 16-17" props. Because of the position of the stock muffler when installed on the engine, using a small lightweight elbow will work as an excellent exhaust diverter.

Karol