H!
The first thing yoy should do is to replace that big .61 twostroke engine...it is simply not suitable for the size CUB your friend has choosen.
Instead a more suitable engine size is a .40 -.46 twostroke, and prop should be a 11x6 -12x4 APC which will give good static pulling power, rapid takeoffs and a quit engine. 5% nitro is good to use, not more!
If the plane is going to hang from the ceiling for a year or more, I recommend that you don't run the engine prior to it being flown. Just put small pieces of paper in the engine intake and exhaust opening so no dust and dirt get in.

Regards!
Jan K
Sweden