Yeah, what he said

There are the invented number runes seen here that you can use. I think you'd be okay using Arabic Numerals as well - a real world example would be Chinese writing using Arabic numerals. For casual writing it would be fine, in a more official capacity it might be harder to explain.

On a sort of related note I'm considering addings words for twenty/thirty/forty/etc., which will help with the issue of written numbers being too long.