You'd need the rotor, stator, regulator, CDI, left side cover and wiring harness from an '88-'00 TW.n The swap can be done in a long afternnon if you're really familiar with this type of work. Basically you replace everything under the left engine cover, install the harness and components, then plug and play. You can also modify the '87 harness for the later system, but you gotta know what you're doing.
Swapping to the '00-current system is a lot more work unless you have a complete parts bike. There are differences, some sniggling, some major, between every one of the connectors, different sizes of clutch and brake swithes, component locations, etc. which make this swap a lot more tedious. The later system is three-phase vs. the earlier single-phase system, the '87 had no provision for running lights etc. You definitely don't want to try this without all the later components and a complete wiring harness unless you're really well versed.
These are the easy routes. Not to make a mountain of a molehill, but making your own harness is either childspay or a complete nightmare, depending entirely on your background.
As for the rest of the platform, late models lack things that old models had and vice-versa, but as far as I'm concerned they'e all a wash. The disc doesn't stop any better than the drum, and I'd gladly trade it for a kickstarter, etc.