The wiring harness and electrical circuits
The electrical circuits are reinforced by the use of relays and better fuse boxes, reliability being the objective here. A modern alternator and belt, a non-leaking battery, uprated lighting are typical additions.
For the more demanding, a modern, programmable PDM (power distribution module) replaces all the fuses and relays assuring trouble-free operation.
And finally, for ease of daily operation and parking, an electrically-assisted power steering can be optioned.