Since I have a keen interest in model engines. Take note of my avatar. I can tell you from experience that 7075-T6 is NOT the way to go. In my experience the engine case should be made from 2024-T6 the head may be of the same material unless you are planning to chrome the cylinder do not use aluminum for the cylinder and piston anodizing will not hold up long enough to give you the proper wear for good compression. If you are planning to actually start and run this engine and build it with minimum cost then a good piece of fine grain cast iron would be in order for the piston and liner. I can tell you that the most critical item in a model engine is the coefficient of thermal expansion. The crankshaft could be made from a piece of 4130 Rc 30 The connecting rod made from the same 2024 ans the case. Sorry, that is all, I can relate at the moment, but I will post several websites that may be of interest within the next day or two got a Boy Scout meeting to go to with my oldest boy.
Note: if you must use aluminum for a piston here are three materials which are excellent for pistons they are high silicon base aluminum but are machinable by the HSM. I know I have made some pistons from one of these Mahle 244 (24% silicon)Peek 30% silicon RSP 30% silicon granted it is not easy but well worth the effort. Any other aluminum without at least 16% silicon would be a waste of time.
Scott