They're supposed to be pressed on. Mine was - it sounds like your bearing is rotating on the collar.
If it were mine, I'd just get some shim stock (unless it is ruined too much with uneven surface), wrap the collar with the shim (take a few test runs for fit) heat the bearing with a propane torch to where you can just hold it and tap it on with a soft-faced mallet.
My bearing was fitted tight to the collar, but I was able to heat it as above (with the collar still cold) and tap it off. Installing the new bearing I used the same process. I have a press, but the odd collar shape made for too much work (yes, I look for the easy way out) and this method works very well for a variety of applications.