You may be familiar with Slate Trigger or Drumagog. They are (expensive) programs that let you take recorded drums and overlay or replace them with drum samples. You can actually do the same thing with a stock Reaper plugin! Granted it's not nearly as powerful, but if you're in a bind and need to do drum replacement this could be just the thing. It's called drumtrigger. Load it up and check it out. Here's a preset that I made.
Threshold depends on the level of your signal. I set this based on a signal that has been normalized.
Set the retrigger interval high enough so you don't get unwanted flams.
Original signal mix, easy enough. 100% if you want to hear the original drum, 0% if you want to replace the drum.
For MIDI note, 38 corresponds to the GM snare specification. Set it accordingly.
Trigger align can be used to nudge the timing.
So, you strap this to your live track. Then you create a MIDI send from the live track to your drum virtual instrument. Give it a try, it works surprisingly well for quick and dirty drum replacement. In case someone sends you awful quality live drums to mix, this gives you a way out.