So melted plastic and electrical systems are never a good combination, as I'm sure you can guess.
Many say that these old bikes have charging systems that could barely keep up with demand even when the bikes were new. Additional resistance in the system shows itself pretty quickly as the charging system cannot keep up. I struggled through this process last summer sorting out a bunch of gremlins of my own.
All the metal connectors inevitably oxidize and corrode with time and as a result cause resistance. Resistance then causes heat, heat causes more resistance, which of course causes more heat, etc etc. It's a vicious loop that leads to bad things. In the case of your melted fuse block, there's enough current, or amperage, running through that part of the circuit to generate enough heat to melt the plastic. I would imagine that your main fuse it the one that melted in. At this point your fuse block is toast and you need to replace it. You can track down a used or NOS one but as long as you're going to the trouble of de/resoldering the fuse block, you should replace it with a modern blade style fuse. More betterer and such. I put a 5 fuse block into my 550, 3 active fuses and two spots for spares. Cost less than $20. It's pretty simple to figure out a way to mount it in the same place as the stock block.
So it's clear, since your fuse block melted, that there was enough corrosion on the blades that hold the fuse to cause some significant resistance. Now there's a whole bunch other of individual wire connections throughout the system, and it's safe to say they are also corroded and causing resistance. It's best to go through every wire connection and lightly abrade the connectors, both the male and female fittings. Then grease them up with dielectric grease and reconnect firmly. Sounds tedious, and it is, but it's really not that bad and you really get to know the bike. A wiring diagram and a multimeter are necessities. And so is some good old fashion patience.
Noodle around this website and there's VERY complete and easy to follow instructions for bringing your charging system back into good health.
http://www.oregonmotorcycleparts.com/Also, to additionally decrease the load on the system you can replace some of the lamp with LEDs. Brake light, all the idiot lights, speedo and tach lights. I did the math once but don't remember the numbers, but that alone drops a couple of amps off the demand.
It's a decent bit of work, but it really helps avoid inevitable goofy electrical problems down the road that can be nightmares to diagnose. I'll post a pic of my *modern* fuse block if you like. That's my 2c, hope it helps and good luck.
