If you really feel into it, there are quite a few things you can do, ranging from cheap to expensive.
First I would recommend changing the bearings, slides, and so on. Sometimes restoring old parts will reduce the drag.
Next, you should look into the carb. The stock butterfly, although it uses less fuel, has less throttle response 76-77 round slide carb. You can also (jetting required) get a brand new mikuni round slide. Some have considered dual carbs, but you will need to find a carb boot that fits the reed cage, which is difficult.
Reeds are another, they don't improve Speed or torque, but they do improve throttle response.
Aaen makes a single pipe for the Exciters, some say there is moderate gain to be had there.
I highly recommend a new camplast .75" track, I have used it for a season, and it has given me fantastic results over the stock track.
If you don't mind tearing it apart, you can port it, and/or bore it out, and use over-sized pistons.
Non-pivot riser, as some feel that the handle bars are too short.
That about all I can think of at the moment, I'm sure there are a few more that I'm missing...
My Current Sled:
1997 Yamaha Vmax XTC 600 Twin ER 6290 MI - Stock... for now...
1979 Yamaha Exciter 440 6042 MI (9825 KM)
1979 Yamaha Exciter 440 7,000 MI (11266 KM)
WAKE-UP AND SMELL THE TWO STROKE
2012-2013 Miles: 1739