Disc brake conversion requires lower left fork leg ( fork tubes and right fork leg are all the same) front wheel with disc, master cylinder with hose, splicing of brake light switch wires (old style plug soldered to new style switch) and ideally the newer kill/starter switch. Your starter button will end up pointing downward with the old style.
Any charging system and wiring harness from a later bike can be retrofitted to an '87, but it's not very cost effective or simple unless you have a complete parts bike. Piece of cake with a parts bike, if everything is there. One '87 CDI unit will cost as much as a wrecked TW, if you shop patiently.