For that Echo hub, any screw on cog will fit. You'll want a 12t on the back, Onza/whatever. Then for the front-freewheel, you'll need specific cranks with the freewheel thread on them. Like these. For the front freewheel, get something like a Tensile, ENO, Monty. Front specific freewheels. Screw it all on, set the chain, and you're sorted.