Welcome to FABO. Lots of us here had the same questions about their 273's.
They are fun little engines and can be built to run pretty good. 318's are cheap and easy to come by and will have as much power as a HP 273. Really the choice is yours. Keep us posted on your build and take lots of photos. We like photos. Check out my 273 build thread to give you an idea what's involved. toolmanmike