Online tool? Wow, you might interesting, we need this kind of such tools available for our Khmer people.
Here you can convert your legacy text (in Limon or ABC font) to Khmer Unicode, just right away.
Of course, currently, we're almost no more articles in legacy fonts since people are using Khmer Unicode already. But it might be sometimes, we need. Here is the tool for online converter in plain text from legacy font to Khmer Unicode.