Can I just replace the valve that’s burnt out in my Smart car?

Applies to: 450, 452

No it will burn out very quickly, it would be like putting a plaster on a broken leg, the problem will happen again (as quickly as 3 weeks, we have seen very unsatisfactory repairs), the only satisfactory option is to replace or get a professionally rebuilt engine. The condition of the bores and the pistons and rings may determine if the engine will need a rebore and new pistons.