Hi Wambat, welcome to the forum!
Following is some good information from our recently departed friend.
Brianlizrdbrth posted this some time ago
All TW's are great bikes. Some (slightly} greater than others, depending on your point of view.
'87's had one-year-only charging systems and ignition modules. The '87 lighting coil was designed to power one 35w bulb at full rpm with A/C current and had a separate circuit for charging the battery that was rectified to D/C current.
Aside from that, they are essentaially no different from an '88-2000 model. They're becoming a parts problem with regard ONLY to the electrical systems.
In 2001, Yamaha giveth us a front disc brake, (but taketh away our kickstarter) slightly more alternator output, a new CV carburetor, and a resetable trip meter.
So basically the bike has had 3 versions of charging system, picked up a disc brake and lost its kickstarter in 24-odd years.
All plastics are interchangeable from year one to present. Lots of lovely Barbie colors over the years if yer into that. lol
The basic motor is the same and all parts will interchange except for the left side covers, which have slightly different castings to accomodate the different charging and CDI systems over the years. Later models had a self-adjusting cam chain adjuster. Intake manifolds, carb boots and cables are a bit different between the early and late carbs but late also fits old, and vice-vera.
Quality control has worsened considerably on the later model motors, so watch for base gasket leaks. Yamaha has issued a Bandaid in the form of an "improved" base gasket, but the real problem stems from random batches of crappy cases. Some suck, some don't.
All front end parts will swap between years. The disc front end differs from the drum only in the left lower legs and that the lower triple tree has a tapped hole for the brake hose mount.
The late model carb drives from the right, early model's cables are on the left.
Swingarms and rear wheels are all the same.
Disc front wheels have thicker spokes and the spoke lengths and lacing pattern is different from a drum. Same spoke count. As far as I'm concerned both are equal in terms of actual braking distances. It's more a matter of "feel" than effectiveness. Pick yer poison here. Drums don't bother me at all. I own both.
The rear brakes are all the same. They blow.
Gas tanks are all the same except later models got a smaller gas cap and Cali models have a fitting for a vapor hose.
Early models have better starter solenoids, relays, and electrical components overall, IMO.
Later models have more output and slightly more sheltered and better protected wiring harnesses.
Lower fork legs lost their drain screws at some point. Early models had them. Huge maintenence issue.
Kickstands, frame gussets and welds are better on the older units. Whether this was neccessary or not is arguable, but they are different.
While this is not a rant against Yamaha, be aware that numerous cost cutting measures like this have been initiated over the years. Most were also accompanied by some fairly significant improvement. Most are insignificant in the real world, but can complicate parts ordering and modifications. Recently some have discovered that the rear muffler mount location has been changed slightly, for example. So even though a 20 year old muffler is identical in every other respect it won't fit the latest and greatest. The good news is that the bike is rock simple to begin with.
The only real interchangeability problems I've encountered have been between the electrical systems, wiring harnesses and switchgear. Lots of variations here, with minor, sniggling changes to the various connectors and components.