an ek or rk chain will do you ok,dont use an o or x ring chain its too wide and will foul on the case at the front sprocket,replace the sprockets at the same time,,your new chain on old sprockets will wear much faster,,you can also buy complete sets of sprockets and chains,google is your friend,lube the chain regularly and dont over tighten it,over tightening is perhaps the most wrongly done thing to a chain short of letting it run dry.danno if your 550 can tear an ek chain in half it must be able to pull the wheel up in third?