You can use any deraillers and any downtube or bar end shifters in FRICTION mode but unless you get vintage SIS shifters from same era you will lose the indexing feature with replacement shifters, if you stick with your original deraillers. You can change out the deraillers and shifters with new ones that are modern 9 or 10 speed but to get them to work in indexed mode you would almost certainly have to change your rear wheel hub to a modern one using a 9 or 10 speed cassette. The most reasonable solution is to replace parts only when they need it. If you need a new freewheel in back you can get a replacement one. If you need a new derailler you can get a replacemnet one new or old just forget about the indexing feature of your old shifters. If you need new shifters just get replacement ones and use them in friction mode and forget about the indexing featuer of your old shifters. If your 6 speed freewheel in back doenst give you an adequate range of gears then you can get a wider range freewheel (you can use a 7 speed) but you will likely need to get a longer derailler.