NOTHING, unless you plan on doing a lot of tricks/jumps/etc. that would involve a lot of falling, in which case a little extra groin clearance would be nice.
If you do mostly road riding, and have clips/clipless pedals, clearance shouldn't be an issue.
If you feel "very comfortable" riding it, and you're getting it practically free, TAKE IT, and chalk this up as a blessing from the bike gods.
(Yes, there are arguments that can be made that track bikes are supposed to be smaller than road bikes, and there's a current vogue for riding smaller frames than necessary (as if saving an inch or two of tubing negated the increased weight of a longer stem/seatpost), etc., etc., etc. but if you're using this outside the velodrome, you should be fine.)
Enjoy it!