My impression would be 1st if the key is broken yes replace it but the answer to the question you should be asking, if you can prime the carb it runs then one would think your not getting fuel, now onto the real question is; the key if the flywheel spun past it then it would be out of time bit then chances of it starting would depend on how far past TDC! Also i have experianced small engines 5hp briggs on push mowers slipping out of time the key is a safe guard againts real damage.
