and preferably remove the basic latin most of them contain
I looked and looked and finally had to cave in and get FontForge, which as the name implies has to be --ugh!-- installed manually --ugh!-- using Terminal --ugh!-- But due to some oversight, I only had to type about two things and it worked on the first attempt. And, once installed, it's got a proper GUI the way God intended.
Like you, I needed a font wrangler so I could strip down an embedded font to only those characters that would actually occur. In my case I was using it for a decorative header so that means [A-Za-z] plus some basic punctuation. Oh, and then I had to fix the size. We're talking third-party font dating from 1991 here ;)
In your case the "stripping-down" detail would make a big difference because if you don't edit your Sinhala font, and if your users have neither Arial nor Helvetica (= they're on Linux*), then that's where their system will get the sans-serif characters.
Matter of fact, I've got a page that says so ;) I thought this all sounded familiar! :: further investigating ::
Drat. I was going to suggest adding DejaVu Sans, which is ubiquitous on Linux and common on Windows (they say nonexistent on Mac but that's a barefaced lie because I've got it), but I remembered wrong. It doesn't cover Sinhala.
According to FontBook, Arial says
You may use this font to display and print content as permitted by the license terms for the product in which this font is included. You may only (i) embed this font in content as permitted by the embedding restrictions included in this font
<snip>
{ (ii) is about printing }
= You are only allowed to do the things you are allowed to do.
If you use a font that's downloadable free-- that is, ahem, from the source, not from a scraper site --then you can pretty well assume embedding is OK, can't you? Especially if it's embedded in a form that makes it unusable as a free-standing font. (The preceding paragraph will come as a great surprise to Tiro Typeworks w/r/t to the Pigiarniq font, but their copyright blurb suggests they're pretty well delusional anyway.) All fonts contain an "embeddable" flag, but unfortunately this seems to refer to pdf embedding, which is a completely different issue since it doesn't create a free-standing .ttf file.
* The font-stack area of codestyle dot org says 99.9% of Windows users and 98.89% of Mac users have Arial-- and 100% of Mac users have Helvetica. But a hair over 30% of Linux users have neither.