Congrats, the D8443 is a great looking box

As for pots and switches some deoxit cleane should bring those back to life. The spring looks to be like from a deck door to spring it open

The line in sucks on these units and were made for recording tapes off of turntable or 8 track. As for tuner string I may be able to help in a few days as I am going to be replacing my belt on deck. Unless someone else chimes in

good luck