'Ja-01' is a font with a custom 'visual encoding'.
That is to say, the sequence of characters really is "bgah;" and it only looks like Tamil to you because the font's shapes for the Latin characters bg look like பெ.
This is always to be avoided, because by storing the content as "bgah;" you lose the ability to search and process it as real Tamil, but this approach was common in the pre-Unicode days especially for less-widespread scripts without mature encoding standards. This application probably predates widespread use of TSCII.
Because it is a custom encoding not shared by any other font, it is very unlikely you will be able to find a tool to convert content in this encoding to proper Unicode characters. It does not appear to be any standard character ordering, so you will have to look at the font (eg in charmap.exe) and note down every character, find the matching character in Unicode and map between them.
For example here's a trivial Python script to replace characters in a file:
mapping= {
u'a': u'\u0BAF', # Tamil letter Ya
u'b': u'\u0BAA', # Tamil letter Pa
u'g': u'\u0BC6', # Tamil vowel sign E (combining)
u'h': u'\u0BB0', # Tamil letter Ra
u';': u'\u0BCD', # Tamil sign virama (combining)
# fill in the rest of the mapping information here!
}
with open('ja01data.txt', 'rb') as fp:
data= fp.read().decode('utf-8')
for char in mapping:
data= data.replace(char, mapping[char])
with open('utf8data.txt', 'wb') as fp:
fp.write(data.encode('utf-8'))
