Where can i find the HTC EVO native camera apk?
Did you take a Nandroid backup or a Titanium backup before flashing? If you have a Titanium backup, you can try restoring the Camera app from there (app only, data can be incompatible). If you didn't backup with Titanium, you could do a new Nandroid backup, restore to the previous one, create the Titanium backup and then restore the new Nandroid backup. After this restore the app from the Titanium backup.

Since this is a system app I can't guarantee the compatibility of the app, but you can try and if it fails/is buggy, you can revert with Nandroid.