Have you driven it yet? If it's brand new with no miles, you need to break it in. HP will increase a little bit, to a point, on brand new cars as you drive them.
I don't know the exact distance needed, but I did the American championship (Indy of course) with a Ford GT LM Spec II Test Car I bought, and it took something like 10 races there to get its max hp.
Mine was 711 with just the racing mods and the oil change before use. i did the removable tuning parts and got like 830 bhp without the 3 engine stages (i wanted go back to 711 after bob used it )
Rebuilding the engine will get you more HP, but you need to race it some moe before you can do that. With all the other tips that will get you your 900+ HP.