4k isn't that bad in fuel economy. Get is sorted/tuned , it will easily give 12-13 km/ltr. No other engine you swap you will go higher than that, so keep it stock. I am attaching a checklist for your convenience.
1- Distributor Overhaul.(Make sure centrifugal and vacuum advance works fine). If possible upgrade to electronic (cdi) distributor.
2- Use W16EX-U Denso or NGK BP5 only.(spark plugs)
3- Get its tappets (Valve lash) adjusted to oem specs.
4- Carburetor would also need to be sorted if you had let any ustad play andha tukka game on your carburetor.