Cost to change is almost prohibitive. Better off keeping an eye out for a replacement, cleaning it really well, selling it, and buying what you want. A Trek 1000 should bring a decent price.
As to your indexing, the old Suntour stuff would index, not flawlessly, but pretty decent. Is your freewheel Suntour, and do the number of cogs match your shifter? And does the RD match the shifters as well? Suntour had various attempts at indexing. I've had several Suntour indexed shifting bikes, and they all shifted OK (but not great).