You have a classic on your hands. You should consider restoring it to working condition with some new Crosman parts, shine it up a little, and enjoy it as is.
You probably need about ten dollars worth of parts (from Crosman) to have it back holding air and shooting again. I would say it needs a new pump cup seal, an inner and outer valve oring, a valve stem, and an oring for the bolt. Capof can walk you through the disassembly and reassembly process. There are some great YouTube videos for it also. If you want to replace the grips or stock, there are lots of options for that too, from 7 or 8 dollars and up. If you take the front grip screw off you will be able to see if it has a brass or an aluminum valve, for that age it should be brass, if so hang on to that valve, it's worth about 25 bucks because all of the newer ones have aluminum valves and the brass valve is more desirable because it doesn't corrode over time like the aluminum might.
In my opinion it is just easier to start with a brand new one if you want to do a lot of modding on a 1377. They can be had for less than forty bucks on Amazon other places.