RE: How much motor to fly this?
Like I said "folks with no imagination"! Yes of course it can be done and is being done by many. Its even being done commercially with products out there that the power source is users choice.
Actually the structural density of many of our modern glow and gas powered RC aircraft has indeed shrunk dramatically in more recent years. One only needs to examine the modern sport scale aircraft even the large gasers. Even these large gas types have far lighter structures than what was typical in Rc structures of perhaps just a few years ago.
This may be due in part to the tendency the not design aircraft to crash at least to the degree that was done in the past but however while yes the standard canned answer is you cannot do it but that is wrong, the more appropriate answer is you must do it right.
John