Hi platinumsmith,
I would say before you do any upgrades make sure all the stock parts you have are in good condition. So start with the ISOs, have they been upgraded to the vernier type? This would make your life a lot easier, remember the bike is 37 years old so any original rubber parts will need replacing by now. Next, your suspension, again if it's on the original shocks they could probably do with reconditioning or replacing (IKON or Hagon?), the same goes for the front forks, you may need seals, bushes and possibly new springs. Also, what tires are you running? If the bike is still on the old hard rubber, again replace those.
Once you have the bike in a good ridable condition you can then decide and make an informed choice of what needs improving or upgrading, there's a thousand ways to "invest" your money in these old tarts, some upgrades make a difference, some less so.
Of course, if you want a 100 BHP fire breathing monster you're better off looking for a modern bike.
I hope this helps
Webby