Hard starting, poor idle and exhaust smell

Much more likely is a fuel problem or an ignition problem.

Check your fuel pressure, fuel flow, fuel injectors, fuel filter, fuel pump, fuel tank before assuming it's the valves.

After that check your ignition wires, your ignition coil or coils, your spark plugs.

A common and easy to run test for valve issues is a compression test. Get a tester and check. A leak-down test can also help but you may need a mechanic to do that.

While some problem with your valves may cause such problems, there are many simpler issues that can cause these that are a lot less expensive to check and repair.