RE: .51 eng. in a .40 size plane
I put a .72 4-stroke in a .40 size war bird. I think you can easily manage that. In fact I just bought a OS .61 off the classifieds for the H9 piper cub .40. I have to, since I'm at 5600 ft. elevation.
p.s. I'm sure your grandpa loves you very much.
edit: totally forgot we were talking about the Sig 4star. This was my first plane and my second one because I loved it so much after my amateur building mistake. The Sig 4Star is a great plane, does very smooth aerobatics and has cub like slow flying characteristics. It's got a long wing chord and allows for landings about as slow as you want. :P I doubt you'll be able to hurt that plane by going too fast, but it will definitely be good to teach you some throttle control.