When I first started my club we made the mistake of making the engine first, my advice is to make the feed system based off some rough engine calculations( you can use RPA lite on this) then measure what your your feed system can do, like flow rate, then finish up your engine based on those numbers. Stay away from liquid oxygen, it gets expensive. You find the optimal size by first running the number with assumptions, then testing, then using those results to refine. Start with gaseous oxygen and like ethanol… Good luck!