the code that draws the captcha image will look like this:
<img src="webassist/captcha/wavt_captchasecurityimages.php?field=Security_Code&noisefreq=15&noisecolor=060606&gridcolor=080808&font=fonts/MOM_T___.TTF&textcolor=040404" alt="Security Code" class="Captcha">
the font querystring:
font=fonts/MOM_T___.TTF
tells it what font file to use.
add your font file to the fonts directory, and edit the font query string to tell it to use the new font file.