If you are saying the kill switch mounted on the engine is not working, you can replace it with a new one. Easy to do since most use solderless terminals on the switch. It is only mounted to the engine since its function for shorting out the ignition coil. you can use almost any switch and locate it where it is easier to access. It does not need to be momentary type. On/Off toggle switch will do the same thing. Best to use one that has an IP67 rating for ingress of contaminants and moisture. The momentary switches common on the engines are not ideal to start with but do the job for a short while (depends on if they get wet with water, oil or grease).
It depends on what model you have. Bajas are easy to reach the on-board kill switch. I could not say the same about others such as short course trucks where you need to reach into the body to find the switch. Nothing wrong with using the remote kill switch to shut off the engine. It is always good to have a secondary in case the wire connection on the remote kill becomes disconnected.