On the top of the trans is a couple of setscrews/allen wrench, which underneath have a spring and a ball bearing, called a detent.
These are what "click" the selector in place when shifting. Remove both screws, and try to pull out the springs, but they likely will have deteriorated with rust. You may be able to get by by just replacing them. It would be best if you could get the balls out too and replace, and you
could introduce some lube in there as well. Otherwise, you will have to remove, dis-assemble, and clean the trans.