The best way is to mic up a drum set and ask a competent drummer to redo the tracks.
Since it's a 4 track, the drums are likely mono (on just one track)? There's no great way to do it inside Reaper automatically. If the drums were recorded on separate audio tracks it would be much simpler.
You could go through the track, hit by hit, and add a midi note for each hit to feed a drum VSTi, if your time is cheap...
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