It is very easy to do yourself. Generally there is one wire from the alternator to the battery. Much simpler than the original setup. Even a regular three-wire setup is simple to wire.
See
Alternator for discussion of Ford Falcon EA swap.

Size comparo - stock alt vs EA

EA mounting diagram
GM alternator swapIn the US the GM 'single wire" alternator used to be the popular swap, since they were the first to have an internal regulator, were commonly available (read, relatively inexpensive) and for those stymied by three wires (BAT, IGN and idiot light), the "one wire" promised simplicity.
The GM alternator is available in versions for 50 amps to over 100 amps.
Datsun 311s.org singlewire: pics, info.
mightymo.org: one-wire swap, troubleshooting, testing, rebuilding alternator
Self-energizing regulator detailsBetter alternatives?Of course, lighter is better, so no point in loading up your 1200 with extra weight if you don't need it. Perhaps a high-efficiency "mini" alternator from a newer Subaru or Toyota would be the way to go: extra amps, but not extra weight.