The chances depends on the condition of the guide itself. If its completly cracked and is only hanging by a thin piece of metal, chances are its going to fall down and eventually some other moving part will pick it up and try to eat it. There are horror stories about this online. Some motors survive it, some dont.
I removed my old KA when i had to replace the valve and spark plug gasket. took me only 1 min to take them out. When i looked at the guide, which is only a piece of steel angle, there was a nice long crack underneath it.

Just unbolt the valve cover, and take a #10 socket wrench and remove the two bolts.
I called up nissan to see if they have a replacement for it, the parts lady said that Nissan discontinued it.
It was simply another mistake that nissan made in the past.
The lower guide is harder to remove, so i didnt bother touching it since the upper has a better chance to break off.