Pilgrim, you should pull up some trim charts and print them out so you have an idea of why your plane is flying funny or needs a lot of radio trim on the controls. I would go look in some of the pattern web sites for the chart. I have several tacked to my walls that are turning yellow with age and I still use them. When I question an engine size I'm not thinking 3-D, I fool around with it but can't/don't really do it. Today we have some very good engines that produce a lot more power then days of old, I was thinking a .55 AX or something on those lines. There are a lot better engine choices out there today in the same size package as the smaller engines of the past. A little extra power is a good thing when learning IMAC or pattern and these are small planes that can be perfect teachers if powered correctly.