the 1.6 diesel hasnt the power to pull the skin off a rice pudding mate andything over 1549cc is more expensive tax so the isuzu 1.7 would come under this, Plus id imagine you'd need some kind of engineers report, the amount of work to get a isuzu in would far outway any savings you'll ever make . i had a mk5 escort 1.8 td with a boost increase and fuel and still got 45mpg and was flat out everywhere, also had a mondeo td for work and driven steady used to knock ojn the door of 50 mpg. they love chip fat and red diesel too lol