4k engines are best and tough engines. Parts are easily available.
whats ur city?
i have a dx81 with 1.3 cc 4K engine. it gives me around 10 kms per litre.
if he has budget why not go for an 7k engine with 5 speed gear box. more power and a much tougher engines. There are efi versions also available in 7k which will give u power as well as economy.
plus they will be bolt on fit i think.