Just ordered my T-Cips 70. Should be ready to go out by mid-August if Sig's timeline holds.
This model will be perfect with a 56 or 62 size four stroke. I'm not sure is people were serious saying they were putting a 20cc gasser in it - much too much for this model and the vibration would be brutal. If you had a light 70 size 4-stroke, that might be okay too.
I had a Hangar 9 Cub (80" wingspan) that was marvelous with my Saito 56. This model is much smaller and lighter than that, so a 56 would be more than enough. Once you get into larger engines, the weight (and tail weight) becomes more of an issue that will detract from the handling and light behavior of the model. Who would put a two-stroke in it anyway? :-)

Chris.