Beware of putting a heavier engine on the Epsilon. I have flown one for almost 10 years. With the OS .91, the CG was spot on. The plane was designed around that engine. I eventually upgraded to the OS 1.20AX, not much heavier than the .91, but I had to move the battery to the tail to balance. Anything heavier on the nose and you'll be adding lead to the tail.
Great airplane. The .91 is really a nice power plant for most flying on this plane. The 1.20 does give that extra oomph on vertical maneuvers. Can power out of vertical snaps and continue climbing all day. But going to the 1.20 (2 stroke), I do have a tendency to tip the 15" props on landing regularly, so be careful of needing any prop larger than that.
Dave