In theory, you could fix the “tempo map” by extracting the MIDI file from the CON (using C3 CON Tools), editing the MIDI (for example in Reaper), and replacing the old MIDI into the CON (using C3 CON Tools).
The difficult part is obviously in Reaper, where the “tempo map” appears as a list of speed markers, usually once per measure. You normally need a visual clue to place markers correctly, and this is provided by the audio file (which you can also extract from the CON). You might be able to correct existing markers if you’re lucky, but editing a pre-existing tempo map can also make it worse, and I think in general people prefer to scrap a bad tempo map and start from zero. You definitely want to watch the original “let’s author” youtube video by pksage to learn how it works: https://www.youtube.com/watch?v=k2rfk21tm1I