Upgrades for economy? Only easy modifications with a positive effect on economy are a properly designed exhaust header, which would have to be custom-designed and a Unichip.
But if you really want to maximize economy, the best first upgrade would be to go for ultra-low rolling resistance tires. Pump them up to about 45 psi to further lower rolling resistance. Even on standard tires, 45+ psi helps keep resistance low.
Easy further upgrades include a partial grille-block. The K10 already has partial grille block compared to the earlier 800cc. You can fashion a full grille block for highway use, but you'll need to remove it for traffic use. Then add an underbody tray and spats to divert air from the wheel wells. Full fairings for the rear tires will lower air resistance, also. Then adjust the brake calipers so that the pads float with greater clearance off the discs, lowering rolling resistance even more.
Then disconnect power steering and airconditioning. If you want to go even further, change the main pulley out so that you're under-driving the alternator.
Hey, since we're compromising the car's utility already, let's change out the side windows and rear window for plexiglass... and switch to lightweight fiberglass or carbon-fiber doors. And since you're re-bodying the car anyway...
With a body like this, an Alto could get well over 40 km/l on the highway.