I know this is a huge bike (displacement wise) compared to an ex250 but I got it cheap have little invested and love the power.
Purchase price of $700
new tires $250... installed
new exhaust $60
tank bag $60
and various sundries brought the total investment to about $1300
this thing gets 60+ mpg at 80+mph and carves curves like a much more expensive bike.
I had the single cylinder version of the Ascot, the FT500 and it was indeed a very nice bike and handled very well, you could lean it over until your elbows nearly dragged without grounding any hard bits.. I'm fond of lightweight, flickable bikes and the Ascot was just that.
You could probably pick up a couple of MPG and make it look even more cool with a pair of bar end mirrors. I bought a pair a while back and like them very much, nicely made of aluminum with convex wide angle mirrors..
The airbox was pretty restrictive on these. You could gain some by reducing restriction with a K&N stock replacement air filter and make sure the intake to the airbox isn't any smaller than the outlet of the filter. Rejet the carbs using an air/fuel meter and a O2 sensor close to the head. With your non-stock exhaust it probably needs a re-jet anyway to maximize gains.
I had a XL600V Transalp, big brother 600cc version of this engine, sweet motor.
the ascot has awsome low end power that is great for gas mileage, removing the air box also remove the velocity stacks that are built in to it and kill that low end power that you want, the duel exhaust will also help with low end power, if you have to up the sizes of the carb jets then you have screwed with the tuning again and can expect a drop in mileage.
best thing you can do for mileage is touring tires that have silica rubber, synthetic rear gear oil as that gear box is a big loss, synthetic engine oil (for motorcycles or you will destroy your clutch!) take advantage of that low end torque and keep your engine speed low, I forget if you can have tubeless tires but they will weight less, then make sure your wheels are perfectly balanced.
Of course you'll have to mount your bars lower, mess with the headlight, etc., etc..
Other more normal things to do:
1) Use a good synthetic grease, and or oil for the shaft.
2) Get the tallest rear tire you can find (for the next one).
3) If you want to really shoot for the moon, explore second member units from the Shadow 750 or 1100, others of this engine family to slow the engine speed. The second member is where the output of trans takes a right angle via gears to the shaft where the countershaft would normally be. Machining may be required.
4) Lastly, use an induction timing strobe light to check to make sure all 4 plugs (dual plug head) are firing. The Transalp had an ignition module that was prone to fail causing only a single plug to fire in each cylinder.
Always liked these 500's, good score. I think the Cycle World test in the day returned over 60mpg, you can do better.
looking at your Khawk it looks like it will interchange but I don't know if it will help. It apears you have the same size wheel as the shadow so the gears are probably the same. My windshield is a Spit fire and it works OK i would have preferred a more upright screen but this seems to do the job OK.
thanks I do like my bike I've put almost 3k miles on it since I bought it.