The FIND mod
by Netclectic -- and currently maintained by Handyman -- is a great place to start. Handyman has recently produced a new version that is amazing for phpBB2. You can contact him and follow his dev progress at his forums
My guess is that the mod itself would need little rewriting, other than switching over from Netclectic's insert_post function (that powers the mod) to the new submit post() that powers phpBB3's posting system.
It seems to me that most of the parsing code wouldn't need to be touched. Also, the ACP side would need to be updated.
I hope this helps in some way.