yeah, most will fit without huge drama. It does depend one which RM bits onto which PE though.
The PE is pretty much just an RM frame with some minor brackets etc. (oh and in plain steel not chrome-moly for all PE's except the first 250B)
of course the earlier tin tanks and tin seat-bases will not happily move to the late-model (80+s) frames.
So you would take an N RM seat and tank and you will probably be able to drop it onto a T/X PE frame.
Fit should not be the issue, more just selecting some rubbers and brackets. For example the PE has brackets to its tanks, whereas the RM had the 'bolt through the frame' rubbers.
Luke