I didn't have much trouble mounting my engine. I used the mount they sent with it, but turned it so that the engine was on its side at a 90 degree angle so that the head stick out the side of the cowl. I used a pitts style muffler with mine. I didn't like the way the stock mounting made you use a wood screw on the one corner and blind nuts and bolts on the other three.

Definitely replace the gear if you go with a bigger engine. The stock ones are very flimsy and won't give you the ground clearence you will need with a bigger prop. My gear didn't even survive the first takeoff.