This can be fun but it can be a trip down a path that can be expensive. I have a Trek 520 from the early 80s, however when parts wear out new ones do not automatically fit. Example I had a worn/bent chainring that I am having replaced but it will probably mean new cranks and bb as well. However the bike rides nicely so I am willing to give it a go. But the old 27 inch wheels may also someday need a rebuild- you see where this leads. If the bike rides well and everything seems to work Ok go for it. If you think you are building a Rivendell you will probably spend about as much on it.