Example build of one in the 1988 magazine article at;
http://mombat.org/MOMBAT/BikeHistoryPages/Fisher.html

Another compounding factor against upgrading is that the bottom bracket is a sealed bearing design that press-fits into the shell (unthreaded), you are pretty much stuck with a square taper BB spindle, a newer hollow axle cranks cannot be retrofitted.

As the frame geometry was designed around a short ridgid fork, trying to put a somewhat modern suspension fork would seriously screw-up the handeling.