If there are spare fuses in the fuse block, these are usually labeled as 'spare'. Replace the burned-out fuse with one of the same current rating. If there are no spares of the same kind, you can purchase a replacement at most any auto parts store. Tip: While you are at the parts store, buy a set of fuses for your fuse block and put these in a zip-lock bag and store in the glove box or other convenient place.

One more question: what effect will removing some of the refrigerant have on the unit? It's so rarely cold in Tampa that I'm sure the tenant will be switching back to A/C tomorrow or the next.

As long as they don't get manipulated much, I would say a long time. Even when the paper comes off, the glue is still there. I have pressed sheets of fusible to fabric and cut from it a year later. But it wouldn't hurt to put something heavy on it like Sharon suggested. beverly

Are you hiring an electrician to do the change out? If so, he will easily be able to determine if your service entrance wires are the proper gauge for a larger service. If the service drop is the original from 1954, then more than likely it will only support 60 amps. However that being said, if you are going through the trouble of changing out the panel, you may as well spring for a new service drop that can support a minimum of 100 amps. Given you have an electric water heater and central a/c, I would go with a 125 or 150 amp service. If your budget is tight, then at the very bare minimum I would change out the sub breaker panel with the damaged buss bar. Sparking and loose connections have the definite possibility of starting a fire. As far as using 20 amps fuses, if you are 100% certain that all wiring on a circuit is #12 gauge, then 20 amps is acceptable. I would be surprised though if the builder used #12 for all circuits in 1954 though. Good luck.