The op rod spring guide (The piece inside the spring ),shim or unitize the gas system, Fiberglass stock, polish the gas piston ,rear site assembly only if yours is mushy,bed the stock,change op rod spring to USGI ,NM ream the flash hider .All these upgrades are better for the money than changing the op rod in my opinion .
If you want to get the bare minimum ,buy a shim set and shim the gas system, replace the spring guide with a NM type, and the stock change (The Chinese wood is very soft and easily compresses) and a USGI op rod spring if you can find one . Here in the U.S. ,you can do this for under $70 .This might cost you a little more in Canada.
HTH
Mike