Yeah, the original said bronze. But, I doubt many people starting out have any bronze, much less enough for 8-10 engines. I think I've only cut bronze once, and that was for a much larger engine. Brass wears quicker, but I don't think it will be a factor for any small engine like this. I have a rocker with a brass shaft and brass bushing/bearing, and it's been run quite a bit with no noticable wear.
I don't think bronze is necessary, and didn't want to exclude anyone who might not have materials on hand or readily (cheaply) available. Mostly, the whole thing could be made from aluminum with steel for the flywheel and brass for the bearing (it's actually a bushing), and have it all work just fine. Naturally, if anyone wants to substitute materials for the team build they can always ask. Making one for yourself? Whatever you have handy will work.