When you say you got rid of the "stock messaging stuff", do you mean you turned off notifications and want to turn them back on, or do you mean you actually removed it? Handcent just runs on top of the stock messaging app. If you remove it, it will break.
If it's the former, just open the stock messaging app->menu->Settings->Check "Received notification". Then just remove Handcent.
If it's the latter, well, I hope you made a nandroid backup. You might be able to find the apk somewhere. Are you running a custom rom or just stock? If you have a custom rom, just reflash it. If not, you can take this opportunity to use one. Just backup everything with titanium backup.