You can use a 12 flathead crank with nut on top then put weights on. Arc sells a billet crank. Huffy44 / tommy Daniels on here or I'm sure many others can balance your complete assembly . Rod, crank, pistion , bolts, clips pulley needed.28 vs 31 has same size valves but has different port design and the 31 has a larger combustion camber. Hope this helps
Owen