I'm seeing this in two possible ways: "Rarely" is the adverb for "to come across" (ie. encounter). "One comes across a dead body very rarely" would be a more direct way of phrasing the sentence. Alternately, it may be possible to interpret "is" as "happens/occurs". Therefore it is appropriate to use "rarely" and not "rare" in this example. Any other thoughts? :)

Yes, you can also write the sentence as, "It is rare that..." In fact, I think the usage of "rarely" is a bit unusual here.

IF we change "rarely" to "rare",is the sentande still right?

yes, it can be with is.. adjective, verb and adverb. IS is used as a linking verb or a connecting verb. examples; adverb-White doves is rarely seen in the city. adjective: She is Beautiful. verb: the turtle is moving slowly.. hope this helps.:)