You will need a substitute as R12 has been banned since 1994.

My recollection is that there are a couple of substitute refrigerants. The one I recall was called R134a. Retrofit to R-134a used to be incompatible with the mineral oil used as lubricants in R12 systems so you had to extract any remaining freon, completely flush the system, replace any rubber hoses, check for leaks, then re-lubricate and refill the system. Doesn't seem like a home handyman thing. Maybe there are some new versions of R134a and the lubricants and the hoses? Maybe jakvis can speak to this?

This not work suitable for the typical home handyman. I do believe It's illegal per Federal law to handle chlorofluorocarbons (CFCs) without certification/training, and possibly illegal to add refrigerant to a leaking system without repairing the leak.

A home freezer (or refrigerator) doesn't have rubber hoses. The system lines are copper or aluminum tubing that's welded-sealed.