Varad, there is a HUGE thread on the cmp hellcat, and its clones on the warbird forum, I can tell you a few things I have found on it as I just finished doing similar to what you are doing...First of all, I think ther were three guys at least that had installed g-23's...which compares probably to a unmodified 25 cc closely. One guy was running a g-26..he said his plane was 15 pounds and flew well. I am using a 25cc mcculloch, it came out at 12 and a half pounds. I haven't flown it yet..it will probably be spring before I do. I think the 31 cc would hinge on how much heavier it is than a 25 cc. If its close it should work. I mounted my elevator servo's 12 inchs from the back of the plane to help balance things. Some of the guys on the warbird thread were even removing 3 inchs on each wing panel at the root, because they thought it was too floaty. That is why the plane is able to support the heavier motor with an unmodified wing. Obviously if you are going gas you need to reinforce and think through the different weight issues on the airframe, I decided to glass the center wing section to be safe. Mechanical retracts are pretty much unuseable with a gas motor I think.. so you will need fixed gear or air retracts .. There is MUCHO info on the warbird thread.. Most of my information here is from what I remember reading on the 57 pages of that thread....

http://www.rcuniverse.com/forum/m_2530290/tm.htm

prepare to be swamped with cmp hellcat talk !