If you already developed the Flash version you won't find an easy way
to reuse code/assets in HTML5. For sure you'll have to convert your
assets into bitmaps.
I doubt EaselJS is good for iOS: it's based on full canvas redraw and
this is efficient only on heavily accelerated canvas engine. But maybe
I'm wrong, so try yourself this EaselJS game on an iPad:
Another lightweight engine option is Crafty.js - animated elements are
placed in separate DIVs which gives you a chance to use iOS' css 3D