I spent a lot of time working on 25-30 year old heat pumps on apartments, we did replace big things until something serious (pump) blew up. Definitely see about patching the tin if that's the only problem. Even Bondo or silicone can work in a pinch.
The problem with 410 conversions is that it is recommended that nothing be salvaged due to incompatibility of the compressor oils. That means that even the lineset would have to come out meaning drywall work (where asbestos may be involved).
Catchya on the Flip Side.....
Emerald Wolf -- would just patch the pan (although I have made new pans out of aluminum roof flashing that worked well too), and then make sure it drains with gravity or add a condensate drain pump.