Suggest not to spend a lot of money updating.
Ride the bike as is; if stuff wears out, then replace/upgrade.
Don't count on aging aunt/uncle to give up the ghost just yet; we are ages 80/77 and are now on our 5th tandem since 1975 and still log thousands of miles a year.
Rode quite a few centuries on our then new 1975 Follis tandem including TOSRV in 1976.
Enjoy the ride TWOgether!
RUdy and Kay/zonatandem