If I completely obliterate my Message Transfer Agent (MTA) service so that the service doesn't start anymore, how can I recreate the MTA database without having to start from scratch?

The MTA requires several core data files, and without those files, your MTA will be obliterated indeed. Those core files include db000000.dat through db000026.dat. (Remember that these filenames are in hexadecimal format; therefore, some files' names will end in letters, not numbers —e.g., db00000c.dat.) You can restore these files either from a backup or from the Bootenv folder on the Exchange Server product CD-ROM. Note that tampering with the MTA's *.dat files can cause you to lose queued mail, so don't restore these files unless you absolutely need to. You might try copying the db*.dat files out of the Mtadata directory before you wipe everything out. After you get the MTA working, you can stop the MTA, copy those files back into their original location, and restart the MTA. In many cases, this action will process the queued messages for delivery.